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/voxel.rs

@@ -2054,6 +2054,7 @@ mod tests {
             temperature_c: Some(18.0),
             moisture_q: 55,
             vegetation_class: VegetationClass::Forest,
+            basin_direction: BasinDirection::South,
         }
     }
 
@@ -2319,6 +2320,7 @@ mod tests {
             temperature_c: Some(18.0),
             moisture_q: 55,
             vegetation_class: VegetationClass::Forest,
+            basin_direction: BasinDirection::South,
         };
         let district_secondary = DistrictProfile {
             morphology_zone: MorphologyZone::AlluvialPlain,
@@ -2332,6 +2334,7 @@ mod tests {
             temperature_c: Some(18.0),
             moisture_q: 55,
             vegetation_class: VegetationClass::Forest,
+            basin_direction: BasinDirection::South,
         };
 
         let (seed, body_id) = (42u64, "blend_cache_test");