test(simulation): golden-seed regression for the Layer 0->1 cascade (#952, D-200)

End-to-end determinism guard. cascade_golden.rs pins two artifacts for a real
committed body heightmap (GJ1c) in one diffable JSON golden
(server/tests/golden/cascade_layer1.json):
- Layer 0: SHA-256 of the source heightmap.png bytes (flips if the Python
  heightmap generator or the file changes)
- Layer 1: the serialized Layer1Output of run_cascade on a 128x64 downsample
  (flips if the Rust drainage/feature/sub-biome code changes)

JSON (not the msgpack discussed in refinement) to match the existing
golden_suite.rs convention and stay diffable — a failure shows what drifted.
UPDATE_GOLDEN=1 regenerates; wired into `make golden-update`.

Adds Serialize/Deserialize to RiverNetwork/DrainageBasin/Layer1Output and a
sha2 dev-dependency.

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

server/Cargo.toml

@@ -42,6 +42,8 @@ gauntlet = []
 # Maintained drop-in fork of the deprecated serde_yaml 0.9 (#966). Test-only:
 # the YAML round-trip in poi.rs and the trait-config fixture in trait_modifiers.rs.
 serde_norway = "0.9"
+# SHA-256 of the source heightmap.png in the cascade golden test (#952).
+sha2 = "0.10"
 
 # ---------------------------------------------------------------------------
 # Explicit test target for the Layer 3 integration module (D-030, ticket #200).