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fix(simulation): PR #202 review round — spill cell always wired + golden truth

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Hoshe finding 1 (live-firing on GJ1c: 2 spill collisions + 1 i==1
collision among 51 Overflow basins): adjacency adjudicated
INSUFFICIENT for the cue — a course's visible anchor is its upstream
cell, so nothing pre-existing belongs to the lake unless wired. The
spill cell (outlet_path[0]) now always gets a real entry: appended
when new, OVERWRITTEN IN PLACE when it collided with an existing river
cell (append would duplicate edge_id; the hydrology solve is the more
authoritative downstream answer for that cell than flat D8
extraction). Interior stop-on-collision stays, now provably safe.
Internal lookup is a dense Vec<Option<usize>>, never iterated (D-010).
Two non-vacuous regression tests prove the cue through build_edges
output; end-to-end on GJ1c all 51 Overflow basins now build a readable
edge (was: one silently missing).

Hoshe finding 2: both doc sites now state the fallback-vs-production
split explicitly (fallback moisture 55: 51/2; production GJ1c moisture
80: 53/53 all-Overflow) — the golden's Endorheic pair is a
fallback-constant artifact, not a fact about GJ1c.

Golden re-regenerated: river_cells 143->192, position-identity diff
purely additive (zero removed, one legitimate in-place overwrite at
the spill-collision cell); attractors/basins/mouths/confluences
byte-identical. Suites: hydrology 26/26, full lib 1943, cascade_golden
1/1, window goldens + believability untouched green.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
This commit is contained in:
jpmschweitzer
2026-07-25 09:04:54 +02:00
co-authored by Claude Fable 5
parent adceb60865
commit 1d2cac9e65
4 changed files with 896 additions and 129 deletions
Download Patch File Download Diff File Expand all files Collapse all files
server/src/atlas/hydrology_equilibrium.rs
+316 -85
View File
@@ -424,44 +424,75 @@ pub fn solve(
/// byte-identical order. No hashing, no `HashMap`, nothing keyed on wall-clock
/// or thread scheduling.
///
/// **Cell selection.** `outlet_path` is "inclusive of both ends" (the basin's
/// `spill_cell` through the downstream terminus) — the spill cell itself is
/// already part of the basin's own lake footprint (a `Lake` gridunit, not a
/// river cell), so it is skipped; every remaining path cell becomes a new
/// river cell whose downstream pointer is the real D8 direction toward the
/// NEXT path cell (mirroring `extract_river_network`'s own `fdir[i]`
/// convention exactly — `outlet_path` is itself a chain of true D8 neighbors,
/// guaranteed by construction: [`cheapest_overflow_path`]'s Dijkstra only
/// ever relaxes D8-adjacent cells). The final path cell's downstream pointer
/// is set from `downstream_target`: [`DownstreamTarget::Sea`] maps to
/// [`RIVER_DOWNSTREAM_MOUTH`] (a real river reaching the sea IS a mouth, the
/// same semantics `extract_river_network` already assigns), and
/// [`DownstreamTarget::Basin`]/[`DownstreamTarget::OpenSpillway`] both map to
/// [`RIVER_DOWNSTREAM_EDGE_DRAIN`] (the course simply ends at the last
/// station — there is no meaningful "direction" once the outlet has reached
/// another lake's footprint or open ground, the same semantics `build_edges`
/// already gives a grid-artifact edge-drain: no further chord to invent).
/// **Cell selection — the spill cell ALWAYS gets a real outflow pointer
/// (PR #202 review, Hoshe finding 1).** `outlet_path` is "inclusive of both
/// ends": index 0 is the basin's `spill_cell` (a `Lake` gridunit at the
/// basin's own boundary — morphology sourcing and river-cell membership are
/// independent, T-1184, so a cell can legitimately be both), index
/// `len - 1` is the downstream terminus. The spill cell is the ONE cell that
/// visually anchors the lake's exit: every course drawn on the map starts at
/// a `RiverEdge::upstream` position ([`crate::atlas::river_course::
/// invent_course`]'s `anchor_a`), so unless the spill cell itself carries an
/// outflow-pointing entry, NOTHING in the served network visually touches
/// this lake — a real `Overflow` basin would read exactly like `Endorheic`
/// (no course leaving the lake), which is the CUE this ticket exists to
/// deliver, not an incidental gap. This is why the spill cell is never
/// skipped, unconditionally: it always gets an entry pointing along
/// `outlet_path` (D8 direction toward `outlet_path[1]`, or straight to the
/// terminus sentinel on a length-1 residual path — see below), whether or
/// not it was already a `river_cells` member. If it WAS already present (a
/// real, if rare, case — verified on GJ1c: 2/51 real `Overflow` basins have
/// a pre-existing-river-cell spill point), its EXISTING entry is
/// overwritten in place (same array index, same position, new downstream
/// direction/class/seaward) rather than appended — appending would create a
/// second `river_cells` entry at the same `(row, col)`, which
/// [`crate::atlas::river_course::build_edges`] would turn into two
/// `RiverEdge`s sharing the same `edge_id` (`pack_cell_id` is a pure
/// function of position, Ruling 2d), corrupting the
/// one-edge-per-upstream-cell invariant `edge_ids_are_unique` guards.
/// Overwriting is correct, not merely safe: the pre-existing pointer was
/// computed by `extract_river_network`'s D8 walk on the ORIGINAL
/// (unfilled) surface, which has no knowledge of the basin's spill
/// direction — the hydrology solve is the more authoritative answer for
/// what water actually does at this specific cell once the basin is full,
/// so it wins.
///
/// **Cells already present in `river_cells` are skipped** (an outlet path can
/// legitimately re-enter the D8-extracted network, e.g. `DownstreamTarget::
/// Basin` chaining through a stretch of terrain the original extraction
/// already classified as a river) — re-adding them would create duplicate
/// entries in the parallel arrays [`crate::atlas::river_course::build_edges`]
/// enumerates by index, corrupting the one-edge-per-upstream-cell invariant
/// (Ruling 2d). The chain still connects visually: the LAST cell before the
/// duplicate correctly points its D8 direction at the existing river cell,
/// which already has its own onward pointer — the course simply continues
/// through machinery that was already there.
/// Every remaining interior path cell (index 1 through `len - 2`) becomes a
/// new river cell whose downstream pointer is the real D8 direction toward
/// the NEXT path cell (mirroring `extract_river_network`'s own `fdir[i]`
/// convention exactly — `outlet_path` is itself a chain of true D8
/// neighbors, guaranteed by construction: [`cheapest_overflow_path`]'s
/// Dijkstra only ever relaxes D8-adjacent cells). The final path cell's
/// downstream pointer is set from `downstream_target`: [`DownstreamTarget::
/// Sea`] maps to [`RIVER_DOWNSTREAM_MOUTH`] (a real river reaching the sea
/// IS a mouth, the same semantics `extract_river_network` already assigns),
/// and [`DownstreamTarget::Basin`]/[`DownstreamTarget::OpenSpillway`] both
/// map to [`RIVER_DOWNSTREAM_EDGE_DRAIN`] (the course simply ends at the
/// last station — there is no meaningful "direction" once the outlet has
/// reached another lake's footprint or open ground, the same semantics
/// `build_edges` already gives a grid-artifact edge-drain: no further chord
/// to invent).
///
/// **`EdgeUnreachable` and empty/single-cell paths contribute nothing** — a
/// path of length ≤ 1 (just the spill cell, already skipped) has no interior
/// cell to assign a direction to, so no new edge is created. This is the
/// correct behaviour, not a gap: a basin whose overflow search exhausted its
/// budget without finding a real terminus has no honest direction to draw a
/// river in, and the module's own guarantee
/// (`overflowing_basin_has_nonempty_outlet_path`) is about `Overflow`
/// basins having a *search path*, not that every search path resolves to a
/// non-trivial extension.
/// **Interior cells (index ≥ 1) already present in `river_cells` STOP the
/// walk** (an outlet path can legitimately re-enter the D8-extracted
/// network, e.g. `DownstreamTarget::Basin` chaining through a stretch of
/// terrain the original extraction already classified as a river) — this is
/// safe (unlike the spill-cell case above) precisely BECAUSE the spill cell
/// always got its own real entry first: the chain from the lake is never
/// silently dropped, only its LATER re-entry into pre-existing machinery is
/// deduplicated. The cell immediately before the collision (which may BE the
/// spill cell itself, on a length-2 path) correctly points its D8 direction
/// at the existing river cell, which already has its own onward pointer —
/// the course continues through machinery that was already there.
///
/// **`EdgeUnreachable` paths and a length-1 residual (spill-only) path still
/// get the spill cell's real entry, just with no interior D8 hop.** A path
/// of length ≤ 1 (`outlet_path == [spill]` — the `EdgeUnreachable`
/// best-effort case, or any basin whose search terminates immediately) has
/// no `outlet_path[1]` to point toward, so the spill cell's downstream
/// sentinel is taken directly from `downstream_target` (the same terminus
/// mapping the multi-cell path's LAST cell uses) instead of a D8 direction —
/// still a real, readable outflow entry, never silently dropped.
///
/// **`river_class` scope note.** Every appended cell is classified `0`
/// (stream — [`crate::atlas::river_course`]'s narrowest, most conservative
@@ -479,17 +510,24 @@ pub fn extend_river_network_with_basin_outlets(
height: u32,
mut network: crate::atlas::body_world_state::RiverNetwork,
) -> crate::atlas::body_world_state::RiverNetwork {
use crate::atlas::body_world_state::{RIVER_DOWNSTREAM_EDGE_DRAIN, RIVER_DOWNSTREAM_MOUTH};
use crate::atlas::body_world_state::RIVER_DOWNSTREAM_MOUTH;
let w = width as usize;
let n = (width as usize) * (height as usize);
// O(1) membership test against the network's pre-existing river cells —
// built once, outside the basin loop (basins can number in the thousands
// on a real body, per the T-1177 population survey: 22,270 basins across
// 267 bodies).
let mut is_river_cell = vec![false; (width as usize) * (height as usize)];
for &(r, c) in &network.river_cells {
is_river_cell[r as usize * w + c as usize] = true;
// Position -> array-index for the network's pre-existing river cells — a
// dense `Vec` (same shape/determinism story as `is_lake`/`basin_of`
// above: O(1) point lookup, never iterated, so there is no D-010
// HashMap-iteration-order concern to even raise). Built once, outside
// the basin loop (basins can number in the thousands on a real body, per
// the T-1177 population survey: 22,270 basins across 267 bodies).
// Doubles as the membership test the interior-cell dedup walk needs; the
// INDEX half is what the spill-cell overwrite case (Hoshe finding 1)
// needs to mutate the correct existing entry in place rather than
// appending a duplicate.
let mut river_cell_index: Vec<Option<usize>> = vec![None; n];
for (idx, &(r, c)) in network.river_cells.iter().enumerate() {
river_cell_index[r as usize * w + c as usize] = Some(idx);
}
// Basins are already stored in ascending `basin_id` order (the `(0..
@@ -504,25 +542,62 @@ pub fn extend_river_network_with_basin_outlets(
else {
continue; // Endorheic — no continuation; absence IS the cue.
};
if outlet_path.is_empty() {
continue; // Degenerate — no spill cell at all to anchor from.
}
// The spill cell's downstream sentinel (fn doc's "Cell selection"
// section): real D8 direction toward `outlet_path[1]` when an
// interior cell exists, otherwise the terminus mapping directly (a
// length-1 residual/EdgeUnreachable path) — either way, ALWAYS a
// real entry, never skipped.
let spill = outlet_path[0];
let (sr, sc) = (spill / w, spill % w);
let spill_sentinel = if outlet_path.len() >= 2 {
let next = outlet_path[1];
let (nr, nc) = (next / w, next % w);
d8_direction_between((sr, sc), (nr, nc), w)
} else {
terminus_sentinel(downstream_target)
};
let spill_seaward = seaward_for(spill_sentinel, sr, sc, RIVER_DOWNSTREAM_MOUTH);
match river_cell_index[spill] {
Some(existing_idx) => {
// Overwrite in place (fn doc: the hydrology solve is the
// more authoritative answer for this cell's true downstream
// direction than the original flat D8 extraction) — never
// append, which would duplicate `edge_id` at this position.
network.river_class[existing_idx] = 0;
network.river_downstream[existing_idx] = spill_sentinel;
network.river_seaward[existing_idx] = spill_seaward;
}
None => {
let new_idx = network.river_cells.len();
network.river_cells.push((sr as u16, sc as u16));
network.river_class.push(0);
network.river_downstream.push(spill_sentinel);
network.river_seaward.push(spill_seaward);
river_cell_index[spill] = Some(new_idx);
}
}
// `outlet_path` is inclusive of the spill cell (index 0) — skip it,
// it's a lake cell, not a river cell. Fewer than 2 entries means no
// interior cell exists to extend from (the EdgeUnreachable
// best-effort `vec![spill]` case, or a degenerate empty path).
if outlet_path.len() < 2 {
continue;
continue; // No interior cell beyond the spill entry above.
}
for i in 1..outlet_path.len() {
let cell = outlet_path[i];
let (r, c) = (cell / w, cell % w);
if is_river_cell[cell] {
if river_cell_index[cell].is_some() {
// Re-entered the pre-existing D8 network (e.g. chained into
// another basin's already-extracted river reach) — the prior
// cell's downstream pointer (set below, on the PREVIOUS loop
// iteration or as the loop's own boundary case) already
// points here, so the chain is visually continuous without
// adding a duplicate entry.
// cell's downstream pointer (the spill entry above, or a
// PREVIOUS loop iteration) already points here, so the chain
// is visually continuous without adding a duplicate entry.
// Safe here (unlike the spill cell) because the spill entry
// above already guarantees the lake's own outflow is never
// silently dropped — this only dedups a LATER re-entry.
break;
}
@@ -533,50 +608,60 @@ pub fn extend_river_network_with_basin_outlets(
let (nr, nc) = (next / w, next % w);
d8_direction_between((r, c), (nr, nc), w)
} else {
match downstream_target {
DownstreamTarget::Sea => RIVER_DOWNSTREAM_MOUTH,
DownstreamTarget::Basin(_) | DownstreamTarget::OpenSpillway => {
RIVER_DOWNSTREAM_EDGE_DRAIN
}
// EdgeUnreachable never reaches here: it only ever
// carries a length-≤-1 path (see `cheapest_overflow_path`),
// already filtered above.
DownstreamTarget::EdgeUnreachable => RIVER_DOWNSTREAM_EDGE_DRAIN,
}
terminus_sentinel(downstream_target)
};
let new_idx = network.river_cells.len();
network.river_cells.push((r as u16, c as u16));
network.river_class.push(0); // stream — fixed default, see fn doc's scope note
network.river_downstream.push(sentinel);
network
.river_seaward
.push(if sentinel == RIVER_DOWNSTREAM_MOUTH {
// A real mouth needs a real seaward neighbor for
// `build_edges`' Mouth-chord synthesis (Ruling 2b's own
// precedent — see `RiverNetwork::river_seaward`'s doc). The
// outlet's own next-cell-below-sea-level is exactly that:
// `cheapest_overflow_path`'s Sea termination condition is
// `original[idx] <= sea_scaled` AT THIS CELL, so `(r, c)`
// itself is already the sub-sea-level neighbor being
// recorded — but `river_seaward` wants the SEAWARD cell, one
// step further than the last land cell. Since this loop only
// ever visits path cells up to and including the terminus,
// and the terminus here has original elevation <= sea_scaled
// by construction, (r, c) IS a below-sea-level cell — use it
// directly (matches `extract_river_network`'s own `(nr, nc)`
// capture, which likewise stores the sub-sea-level neighbor
// itself, not a cell beyond it).
(r as u16, c as u16)
} else {
(0, 0)
});
is_river_cell[cell] = true;
.push(seaward_for(sentinel, r, c, RIVER_DOWNSTREAM_MOUTH));
river_cell_index[cell] = Some(new_idx);
}
}
network
}
/// Terminus sentinel for a [`BasinOutcome::Overflow`]'s downstream end, from
/// [`DownstreamTarget`] — shared by both the spill-cell entry (a length-1
/// residual path with no interior cell to point at) and the last interior
/// path cell's entry (the common multi-cell case).
/// [`DownstreamTarget::Sea`] maps to [`RIVER_DOWNSTREAM_MOUTH`] (a real
/// river reaching the sea IS a mouth, the same semantics
/// `extract_river_network` already assigns); every other variant
/// ([`DownstreamTarget::Basin`], [`DownstreamTarget::OpenSpillway`],
/// [`DownstreamTarget::EdgeUnreachable`]) maps to
/// [`RIVER_DOWNSTREAM_EDGE_DRAIN`] — no further chord to invent once the
/// outlet has reached another lake's footprint, open ground, or exhausted
/// its search budget.
fn terminus_sentinel(target: &DownstreamTarget) -> u8 {
use crate::atlas::body_world_state::{RIVER_DOWNSTREAM_EDGE_DRAIN, RIVER_DOWNSTREAM_MOUTH};
match target {
DownstreamTarget::Sea => RIVER_DOWNSTREAM_MOUTH,
DownstreamTarget::Basin(_)
| DownstreamTarget::OpenSpillway
| DownstreamTarget::EdgeUnreachable => RIVER_DOWNSTREAM_EDGE_DRAIN,
}
}
/// [`RiverNetwork::river_seaward`] value for a cell whose downstream
/// sentinel is `sentinel` — a real seaward neighbor when `sentinel ==
/// mouth_sentinel` (see [`extend_river_network_with_basin_outlets`]'s doc on
/// why `(r, c)` itself, not a cell beyond it, is the correct seaward value
/// here — mirrors `extract_river_network`'s own `(nr, nc)` capture), the
/// unreadable `(0, 0)` placeholder otherwise (matches every non-MOUTH
/// `river_seaward` entry across the codebase).
fn seaward_for(sentinel: u8, r: usize, c: usize, mouth_sentinel: u8) -> (u16, u16) {
if sentinel == mouth_sentinel {
(r as u16, c as u16)
} else {
(0, 0)
}
}
/// The D8 direction index `k` (matching [`d8_offset`]'s table) such that
/// stepping from `(r, c)` by `d8_offset(k)` (with horizontal wraparound,
/// [`crate::atlas::drainage`]'s convention) reaches `(nr, nc)`. Panics if the
@@ -1536,6 +1621,152 @@ mod tests {
);
}
/// PR #202 review, Hoshe finding 1 — the i==1 collision regression,
/// asserted NON-VACUOUSLY on the actual D-227 amendment (4) cue: when
/// `outlet_path[1]` (the FIRST interior cell after the spill) is already
/// a pre-existing river cell, the basin's Overflow status must still be
/// READABLE via `build_edges` — a course must leave the lake. Before the
/// fix, this exact scenario made the loop `break` on its first
/// iteration having pushed zero cells for the basin, silently making a
/// real Overflow basin indistinguishable from Endorheic.
#[test]
fn overflow_basin_survives_an_i_equals_one_collision_non_vacuously() {
let result = overflow_bowl_result();
let overflow = result
.basins
.iter()
.find(|b| matches!(b.outcome, BasinOutcome::Overflow { .. }))
.expect("fixture sanity: at least one Overflow basin");
let BasinOutcome::Overflow { outlet_path, .. } = &overflow.outcome else {
unreachable!()
};
assert!(
outlet_path.len() >= 2,
"fixture sanity: outlet_path must have an interior cell at index 1 \
to pre-seed as the i==1 collision"
);
let spill = outlet_path[0];
let collision_cell = outlet_path[1]; // the exact i==1 collision position
let (sr, sc) = (spill / 64, spill % 64);
let (cr, cc) = (collision_cell / 64, collision_cell % 64);
// Pre-seed the network with ONLY the i==1 cell as a native river
// cell (pointing at some unrelated existing direction) — the spill
// cell itself is NOT pre-existing, isolating this as the exact
// "first interior cell collides" scenario Hoshe's finding names.
let mut network = empty_network();
network.river_cells.push((cr as u16, cc as u16));
network.river_class.push(1);
network
.river_downstream
.push(crate::atlas::body_world_state::RIVER_DOWNSTREAM_EDGE_DRAIN);
network.river_seaward.push((0, 0));
let after = extend_river_network_with_basin_outlets(&result, 64, 32, network);
// The spill cell must have its OWN entry — this is the actual fix:
// before it, nothing was ever pushed for this basin at all.
let spill_idx = after
.river_cells
.iter()
.position(|&cell| cell == (sr as u16, sc as u16))
.expect(
"the spill cell must have a real river_cells entry even when \
outlet_path[1] collides with a pre-existing river cell — this \
is the exact bug PR #202 flagged: a silent break at i==1 must \
never erase the basin's own outflow anchor",
);
// Its downstream pointer must be a real D8 direction (not a sentinel)
// toward the collision cell, since outlet_path[1] IS the next hop.
assert!(
after.river_downstream[spill_idx] < 8,
"the spill cell's downstream pointer must be a real D8 direction \
toward outlet_path[1], not a terminus sentinel"
);
// Non-vacuous on the cue itself: build_edges must produce a real
// edge whose upstream IS the spill cell — an overflow lake's exit
// river must be readable from the served network, not just present
// as raw unreachable array data.
let edges = crate::atlas::river_course::build_edges(&after);
assert!(
edges.iter().any(|e| e.upstream == (sr as u16, sc as u16)),
"an Overflow basin's spill cell must produce a real RiverEdge even \
when its first interior outlet cell collides with a pre-existing \
river cell — otherwise this basin is visually indistinguishable \
from Endorheic, which is exactly the cue this ticket must not break"
);
}
/// The sibling collision case: the SPILL CELL ITSELF (not an interior
/// cell) is already a pre-existing river cell — verified to occur on
/// real GJ1c data (2/51 real Overflow basins). The pre-existing entry's
/// downstream pointer must be overwritten to point along the outlet
/// path (the hydrology solve is the more authoritative answer for this
/// cell's true direction), not left pointing wherever the original flat
/// D8 extraction guessed — and it must not create a duplicate
/// `river_cells` entry at the same position.
#[test]
fn overflow_basin_overwrites_a_pre_existing_spill_cell_in_place() {
let result = overflow_bowl_result();
let overflow = result
.basins
.iter()
.find(|b| matches!(b.outcome, BasinOutcome::Overflow { .. }))
.expect("fixture sanity: at least one Overflow basin");
let BasinOutcome::Overflow { outlet_path, .. } = &overflow.outcome else {
unreachable!()
};
assert!(
outlet_path.len() >= 2,
"fixture sanity: needs an interior cell"
);
let spill = outlet_path[0];
let (sr, sc) = (spill / 64, spill % 64);
// Pre-seed the network with the SPILL cell as a native river cell,
// pointing in an unrelated direction (simulating the original flat
// D8 extraction having already claimed this cell for its own,
// wrong-for-the-lake reasons).
let mut network = empty_network();
network.river_cells.push((sr as u16, sc as u16));
network.river_class.push(2); // deliberately a different class
network.river_downstream.push(0); // deliberately a real but likely-wrong direction
network.river_seaward.push((0, 0));
let after = extend_river_network_with_basin_outlets(&result, 64, 32, network);
// Exactly one entry at the spill position — overwritten, not duplicated.
let occurrences = after
.river_cells
.iter()
.filter(|&&cell| cell == (sr as u16, sc as u16))
.count();
assert_eq!(
occurrences, 1,
"the spill cell must be overwritten in place, never duplicated"
);
let spill_idx = after
.river_cells
.iter()
.position(|&cell| cell == (sr as u16, sc as u16))
.unwrap();
assert!(
after.river_downstream[spill_idx] < 8,
"the overwritten spill cell must point along the real outlet path, \
not retain whatever direction the pre-existing entry had"
);
// Non-vacuous on the cue: the overwritten spill cell must still
// build into a real, readable RiverEdge.
let edges = crate::atlas::river_course::build_edges(&after);
assert!(
edges.iter().any(|e| e.upstream == (sr as u16, sc as u16)),
"an overwritten spill cell must still produce a real RiverEdge — \
the lake's outflow must remain readable"
);
}
#[test]
fn d8_direction_between_matches_the_offset_table() {
// Round-trip sanity: for every direction in the D8 table, stepping
server/src/atlas/layer1.rs
+17
View File
@@ -83,6 +83,23 @@ pub struct Layer1Output {
/// used ONLY by [`run_layer1`]'s two-arg form; [`run_layer1_with_moisture`]
/// (called by every production site that has real `BodyParams` in scope) never
/// reaches this constant.
///
/// **This constant is NOT what a player sees on any real body (PR #202
/// review, Hoshe finding 2) — never cite a basin's Overflow/Endorheic split
/// at this moisture value as representative of production.** The
/// endorheic-vs-overflow decision (`hydrology_equilibrium::is_endorheic`)
/// gates on `moisture_q <= ENDORHEIC_MOISTURE_CEILING (60)`; this fallback
/// (55) sits just BELOW that ceiling, so a fallback-path solve can show
/// Endorheic basins that would never occur on the real body. Concretely, on
/// GJ1c (`hydrosphere: liquid_water`, `atmosphere: standard` →
/// `derive_moisture_ceiling_q` = 80, well ABOVE the ceiling): the fallback
/// path (55) splits its 53 real basins 51 Overflow / 2 Endorheic, while the
/// PRODUCTION path (80, via `run_layer1_with_moisture` at a real call site)
/// is 53/53 all-Overflow on the identical geometry — moisture is the only
/// variable that moves (`filled_scaled`/basin geometry itself is
/// moisture-independent). Any doc, golden comment, or report citing a
/// basin-outcome ratio must state which moisture path produced it — see
/// `tests/cascade_golden.rs`'s own re-pin note for the worked example.
const DEFAULT_HYDROLOGY_MOISTURE_Q: i32 = 55;
/// Run the Layer-1 topography pipeline for a single body.