refactor(simulation): harden #952 cascade per QA + architecture review

Addresses the Hoshe (QA) + Tyre (architecture) review of the SeedChain/cascade
work:

- Golden was pinning an empty river network (128x64 produced 0 river cells).
  Bumped to 256x128, where GJ1c yields a real network (93 river cells, 19
  mouths) — Layer 1's rivers are now actually guarded, not just attractors.
- SeedChain::for_body(world_seed, body_id) + fnv1a_64: the single canonical
  body_id(String) -> u64 path (FNV-1a, the repo convention), so callers can't
  derive divergent worlds from the same seed via different ad-hoc hashes. The
  golden now uses it.
- Stability guards: seed_domain_discriminants_are_pinned test (CI fails if a
  SeedDomain tag is renumbered); AttractorType gains #[repr(u8)] + explicit
  discriminants (it's cast as a sort key in features.rs).
- Tests: SeedChain::root(0) non-degenerate; run_cascade error path (missing
  file -> Err, not panic).
- Comments: clarified the id=0 derivations (sibling separation is caller-side
  via the per-district/quarter chain; #957 threads the index), tightened the
  all_district_types reachability comment (it pins the seed-0 sequence, not a
  probabilistic claim), and noted the golden's WORLD_SEED is cosmetic at
  Layers 0-1 + the x86_64 f32 capture caveat.

Deferred with reason: the run_cascade -> CascadeInputs struct refactor (Tyre)
is left for #954 — designing Layer-2's context shape now would be later-phase
detail, and there's a single caller to migrate then. EntityRng keeps its
domainless combine (migrating is stream-changing) — noted in D-224.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

server/src/simulation/generator.rs

@@ -371,22 +371,27 @@ pub enum TerritorialStatus {
 
 /// The type of terrain feature that attracts settlement placement.
 /// Source: D-195, D-209
+// `#[repr(u8)]` with explicit discriminants: `AttractorType` is cast `as u8` as
+// the primary sort key for the attractor list (`features.rs`), so the layout is
+// load-bearing — reordering would change attractor ordering and the cascade
+// golden. Append new variants; never renumber or reorder existing ones.
 #[derive(Serialize, Deserialize, Clone, Copy, Debug, PartialEq, Eq)]
+#[repr(u8)]
 pub enum AttractorType {
     /// Where a river meets sea level or coastline. Historically high-value.
-    RiverMouth,
+    RiverMouth = 0,
     /// Proximity to coast without a river mouth. Port access.
-    CoastalAccess,
+    CoastalAccess = 1,
     /// Where a river crosses a topographic saddle or confluence point.
-    RiverCrossing,
+    RiverCrossing = 2,
     /// Local elevation minimum; flat, arable, sheltered.
-    ValleyFloor,
+    ValleyFloor = 3,
     /// Saddle point between adjacent drainage basins; controls a mountain pass.
-    PassEntrance,
+    PassEntrance = 4,
     /// Adjacent to a lake polygon.
-    LakeShore,
+    LakeShore = 5,
     /// Flat terrain away from all other attractors; fallback for plains settlements.
-    PlainCenter,
+    PlainCenter = 6,
 }
 
 /// Fine-grained terrain classification carried by each `GeographicAttractor`.