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/src/atlas/layer1.rs

@@ -21,9 +21,10 @@ use crate::atlas::features::{self, TerrainAnalysis};
 use crate::atlas::heightmap::BodyHeightmap;
 use crate::atlas::subbiome;
 use crate::simulation::generator::{AttractorType, GeographicAttractor};
+use serde::{Deserialize, Serialize};
 
 /// Full Layer-1 result for one body.
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct Layer1Output {
     pub body_id: String,
     pub river_network: RiverNetwork,