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>

Makefile

@@ -209,8 +209,9 @@ golden-diff:
 	fi
 
 golden-update:
-	@echo "Regenerating golden file..."
+	@echo "Regenerating golden files..."
 	@cd server && UPDATE_GOLDEN=1 cargo test --test golden_suite -- proof_room_tick_10_matches_golden
+	@cd server && UPDATE_GOLDEN=1 cargo test --test cascade_golden
 	@git add server/tests/golden/
 	@echo "--- Golden file updated and staged ---"
 	@echo "Review with: git diff --cached -- server/tests/golden/"