fix(simulation): PR #197 review round — real seaward chord for Mouth edges (Hoshe #1/#2, Tyre 1/2)

The batch's real bug: extract_river_network computed the seaward
neighbor (nr,nc) to decide the MOUTH sentinel then discarded it, and
build_edges placeholder-pointed mouth edges at themselves — zero
chord, invent_course's degenerate 1-point return, resolve_mouth_
terminus dead code on real data, ALL real mouths resolving None, and
(because Ruling 3g retired the D/Q clip on the promise of real
termini) mouths vanishing at District/Quarter. The second instance of
the threw-away-the-answer anti-pattern Ruling 2b fixed for interior
pointers. Fix: river_seaward: Vec<(u16,u16)> on RiverNetwork
(additive, serde-default, parallel array; meaningful only at MOUTH
entries), captured in the same extraction pass; build_edges gives
Mouth edges the real one-D8-step chord. Permanent acceptance:
all_real_gj1c_mouths_resolve_to_mouth_terminus_not_none — 3/3, revert-
verified failing at the golden's first mouth (38,47). Mouth golden
coverage added (river_course_golden gains district_mouth/quarter_mouth
samples + non-degeneracy test; the previously Interior-only filter
gap closed). Tyre 1: the land-probe's dead dx/len*len arithmetic
replaced — station_spacing_m threaded through the crop path, probe
steps one real cell spacing, comment reconciled. Tyre 2: boxing
comment reattributed to variant-size balancing (Layer1Output retention
lives on TerrainAnalysisCache, not BodyWorldState). Hoshe #4:
cascade_golden's doc now states the attractor cascade accurately
(count-parity, not byte-identity — water_dist seeds from mouths).
Full cargo test green; goldens re-pinned deliberately; bench +3.0%.

Tickets: T-1170

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

server/src/atlas/gen_queue.rs

@@ -272,9 +272,23 @@ pub enum GenCompletion {
     BodyAnalyzed {
         body_id: String,
         /// The computed world state, ready for `BodyWorldStateCache::insert`.
-        /// Boxed: `BodyWorldState` grew past clippy's large-enum-variant
-        /// threshold when it began retaining `Layer1Output` (T-1170 Ruling 4b)
-        /// — the box keeps the enum small while the state moves by pointer.
+        /// Boxed to keep `GenCompletion` variant sizes balanced — the same
+        /// discipline as `SkeletonGenerated`/`ChunkFilled`/`WindowDerived`
+        /// below, all boxed for the same reason (`GenCompletion`'s overall
+        /// size is bounded by its largest unboxed variant; a `BodyWorldState`
+        /// field carried by value here would force every other variant to
+        /// pay for its full stack size on every match/move).
+        ///
+        /// **Not this variant's own recent growth** (T-1170 PR #197 review,
+        /// Tyre issue 2 — the prior comment here overattributed the boxing
+        /// rationale): `BodyWorldState` itself only grew by
+        /// `RiverNetwork`'s two new `Vec` fields (`river_downstream`,
+        /// `river_seaward` — a few bytes/river-cell, Ruling 2a / Hoshe #1).
+        /// The much larger `Layer1Output` retention (T-1170 Ruling 4b) lives
+        /// on `TerrainAnalysisCache` — a queue-scoped struct further down
+        /// this file, never part of `BodyWorldState`/`BodyWorldStateCache` at
+        /// all. The box here predates T-1170 and stays for the pre-existing
+        /// variant-size-balancing reason, unrelated to this ticket's growth.
         state: Box<BodyWorldState>,
     },
     SkeletonGenerated {