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/atlas/skeleton_gen.rs

@@ -245,6 +245,8 @@ fn derive_layout_mode(
 /// scaled to the ±16 sim tile maximum from `block_irregularity::max_offset_sim_tiles`.
 fn organic_placements(irregularity: f32, chain: SeedChain) -> [[BlockPlacement; 4]; 4] {
     let max_offset = (irregularity * 16.0) as i16;
+    // id = 0: one placement pass per district. Sibling separation comes from the
+    // distinct `chain` per district/quarter (caller-derived; #957), not the id.
     let mut lcg = chain.derive(SeedDomain::Block, 0).atlas_rng();
 
     // Build the 2D array using a flat closure to keep things readable.