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/plugin.rs

@@ -644,6 +644,7 @@ mod tests {
         use crate::atlas::district_profile::{
             DistrictProfile, GlaciationGrade, PrecipitationClass, VegetationClass,
         };
+        use crate::atlas::scale::BasinDirection;
         use crate::simulation::generator::MorphologyZone;
 
         // A Corporate archetype placement in a Fjord district.
@@ -672,6 +673,7 @@ mod tests {
                 temperature_c: Some(8.0),
                 moisture_q: 55,
                 vegetation_class: VegetationClass::Scrub,
+                basin_direction: BasinDirection::North,
             },
         );
 
@@ -853,6 +855,7 @@ mod tests {
         use crate::atlas::district_profile::{
             DistrictProfile, GlaciationGrade, PrecipitationClass, VegetationClass,
         };
+        use crate::atlas::scale::BasinDirection;
         use crate::atlas::skeleton_gen::generate_quarter_skeleton;
         use crate::simulation::generator::AccessKind;
 
@@ -881,6 +884,7 @@ mod tests {
                 temperature_c: Some(5.0),
                 moisture_q: 35,
                 vegetation_class: VegetationClass::Barren,
+                basin_direction: BasinDirection::North,
             },
         );