feat(simulation): cache TerrainAnalysis + derive basin_direction from the D8 thalweg (T-1044, T-1047)

T-1044: run_layer1 now returns TerrainAnalysis (carried transiently on
CascadeSnapshot, dropped after the district + road-graph passes), eliminating the
redundant per-body drainage::analyze + TerrainAnalysis::analyze re-run flagged by
PERF/TODO(T-1044). Not persisted on the LRU-cached state (D-203/T-1048 size concern).

T-1047: basin_direction is now derived from the real D8 thalweg. run_layer1
aggregates a per-district dominant D8 direction from the live fdir grid (carried
transiently on DrainageResult), threaded via Layer1Output.district_basin_dirs ->
derive_all_districts -> DistrictProfile.basin_direction; derive_chunk_context reads
it directly. Removed the false derive_basin_direction (it branched on ocean_fraction_q
then read seed bits despite a doc comment claiming an elev_q/slope_q D8 proxy) +
corrected the module contract. D-239 §8 (D8 thalweg) now actually honoured.

1559 tests pass; golden byte-identical (district_basin_dirs is #[serde(skip)], transient).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

server/src/atlas/layer_proxy.rs

@@ -145,6 +145,13 @@ pub fn handle_atlas_request(
             // working grid all Layer-1 positions are expressed in (#960).
             grid_w: state.heightmap_width,
             grid_h: state.heightmap_height,
+            // district_basin_dirs is transient — it is aggregated during run_layer1
+            // and consumed by derive_all_districts before being stored on
+            // BodyWorldState. When reconstructing Layer1Output from the cache for
+            // the client response, the per-district direction is already encoded in
+            // DistrictProfile.basin_direction (BodyWorldState.districts) and is not
+            // needed again here. Supply an empty map.
+            district_basin_dirs: std::collections::BTreeMap::new(),
         };
         let district_grid = build_district_grid(state);
         return AtlasLayerResponse {
@@ -271,6 +278,7 @@ mod tests {
             temperature_c: Some(10.0),
             moisture_q: 50,
             vegetation_class: VegetationClass::Barren,
+            basin_direction: crate::atlas::scale::BasinDirection::default(),
         };
         let mut state = BodyWorldState {
             body_id: "GJ1c".into(),