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/tests/cascade_golden.rs

@@ -14,22 +14,27 @@
 //!
 //! Regenerate after an intended change:
 //!   UPDATE_GOLDEN=1 cargo test --test cascade_golden
+//!
+//! Golden captured on x86_64. The downsample and sub-biome cost use f32, so a
+//! different architecture could in principle round differently — regenerate
+//! per-arch if CI ever moves off x86_64.
 
 use std::path::PathBuf;
 
 use serde_json::{json, Value};
 use settled_reach_server::atlas::cascade::{run_cascade_from_heightmap, CascadeLayer};
 use settled_reach_server::atlas::heightmap::load_heightmap_png;
-use settled_reach_server::seed::{SeedChain, SeedDomain};
+use settled_reach_server::seed::SeedChain;
 use sha2::{Digest, Sha256};
 
 /// Source heightmap — a real committed body, relative to the server manifest dir.
 const SOURCE_HEIGHTMAP: &str = "../wiki/star-systems/GJ-1/bodies/GJ1c/heightmap.png";
-/// Downsample target: small enough for a compact golden, large enough for real
-/// drainage/feature structure.
-const DOWNSAMPLE: (u32, u32) = (128, 64);
-/// World seed for the run. Layers 0–1 are RNG-free; this is carried for the
-/// SeedChain contract (D-224).
+/// Downsample target: large enough that GJ1c's drainage produces a real river
+/// network (not just attractors), small enough for a compact golden.
+const DOWNSAMPLE: (u32, u32) = (256, 128);
+/// World seed for the run. Cosmetic here — Layers 0–1 are RNG-free, so changing
+/// it does not change the golden; it is carried only to exercise the SeedChain
+/// contract end-to-end (D-224). Seed-sensitivity gets pinned once Layer 3+ lands.
 const WORLD_SEED: u64 = 42;
 const GOLDEN: &str = "tests/golden/cascade_layer1.json";
 
@@ -49,7 +54,7 @@ fn cascade_layer0_to_1_matches_golden() {
 
     // ── Layer 1 — downsample, then run the cascade to topography. ───────────
     let small = heightmap.downsample(DOWNSAMPLE.0, DOWNSAMPLE.1);
-    let body_seed = SeedChain::root(WORLD_SEED).derive(SeedDomain::Body, 1);
+    let body_seed = SeedChain::for_body(WORLD_SEED, "GJ1c");
     let snapshot = run_cascade_from_heightmap(body_seed, small, CascadeLayer::Topography);
     let layer1 = snapshot.layer1.expect("Layer 1 ran");