fix(simulation): one absolute-metre derive core (D-256, T-1174)

derive_district_profile is now a thin wrapper over the shared
derive_at_metres_with_riparian core at survey-cell-centre world metres
— one derive core, two position sets. The batch pseudo-grid and the
true D-243 district grid were two coordinate systems sharing one bare
(i32,i32) type; the new SurveyCellPos newtype re-keys every batch
product (BodyWorldState.districts, Layer1Output.survey_basin_dirs) so
the compiler rejects cross-namespace passing.

Fixes two latent same-position divergences the T-1174 investigation
surfaced: three inconsistent latitude conventions collapse into the
core's single inverse mapping, and the region-climate baseline now
floor-divides true world metres instead of collapsing the whole body
onto region (0,0)'s baseline — batch climate becomes latitude/region
graded (D-245 direction: every changed believability metric increased).

Binding preservations per D-256(c): basin_direction rides a post-call
override with the true L1 D8 survey-cell aggregate (layer1's map
re-keyed to SurveyCellPos, identity lookup — a floor-divide lookup
against the pseudo-keyed map would have silently defaulted every cell
North); the riparian verdict comes from near_perennial_water_at, never
the empty-slice default (which would have flipped riverside
vegetation_class).

Quarter-skeleton morphology_zone now resolves at the settlement's
exact world position via derive_at_metres at work-item execution
(where TerrainAnalysisCache lives), replacing the survey-cell-centre
map lookup (D-256(d)); settlement_district_pos fixed to true-district
floor-division in passing (same doc/impl mismatch class). Second
pixel-vs-metre conflation fixed in aliveness_probe's anchor-walk math.

Window path byte-unchanged (window_derivation_golden 6/6 byte-
identical); derivation_harness golden untouched; believability golden
regenerated. Full lib + integration suites green.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

server/tests/believability_harness.rs

@@ -26,7 +26,7 @@
 //! `cascade_golden`).
 
 use settled_reach_server::atlas::believability::{
-    analyze, cascade_for_body, evaluate_criteria, seed_to_u64, BelievabilityReport,
+    analyze, cascade_snapshot_for_body, evaluate_criteria, seed_to_u64, BelievabilityReport,
 };
 
 const GOLDEN_FILE: &str = "tests/golden/believability.json";
@@ -42,8 +42,18 @@ const VALIDATION_BODIES: &[(&str, &str, &str)] = &[
 /// Run the cascade + analyze for one body, or `None` if its committed data is absent.
 fn report_for(body_id: &str, seed: &str) -> Option<BelievabilityReport> {
     let world_seed = seed_to_u64(seed);
-    match cascade_for_body(world_seed, body_id) {
-        Ok(bws) => Some(analyze(world_seed, body_id, &bws.districts)),
+    match cascade_snapshot_for_body(world_seed, body_id) {
+        Ok((snapshot, params)) => {
+            let bws = snapshot.into_body_world_state();
+            Some(analyze(
+                world_seed,
+                body_id,
+                &bws.districts,
+                bws.heightmap_width,
+                bws.heightmap_height,
+                params.body_radius_km,
+            ))
+        }
         Err(e) => {
             eprintln!("[believability] SKIP {body_id}: {e}");
             None

server/tests/derivation_harness.rs

@@ -1927,7 +1927,7 @@ fn derive_profile_for_body(params: &BodyParams) -> DistrictProfile {
         seed,
         params,
         &ta,
-        (0, 0),
+        scale::SurveyCellPos(0, 0),
         8,
         &climate,
         "test_body",

server/tests/gen_fixtures.rs

@@ -578,7 +578,7 @@ fn generate_atlas_layer_response_fixtures() {
         }],
         grid_w: 512,
         grid_h: 256,
-        district_basin_dirs: std::collections::BTreeMap::new(),
+        survey_basin_dirs: std::collections::BTreeMap::new(),
     };
     // T-960 §1/§2: a small populated RoadGraphLayer + SettlementLayer, one
     // settlement (a capital) connected to one waypoint-free short edge.

server/tests/golden/believability.json

@@ -6,7 +6,7 @@
     "voxel_sampled_districts": 64,
     "contrast": {
       "moisture_q": {
-        "min": 16,
+        "min": 17,
         "max": 97,
         "distinct": 74
       },
@@ -23,7 +23,7 @@
       "ocean_fraction_q": {
         "min": 0,
         "max": 100,
-        "distinct": 42
+        "distinct": 44
       },
       "morphology_zones": 9,
       "vegetation_classes": 4,
@@ -37,7 +37,7 @@
       "drainage_samples": 0,
       "drainage_monotonic": 0,
       "vegetation_samples": 64,
-      "vegetated_districts": 5
+      "vegetated_districts": 8
     }
   },
   {
@@ -69,7 +69,7 @@
       "morphology_zones": 6,
       "vegetation_classes": 2,
       "terrain_materials": 2,
-      "voxel_relief_m": 21,
+      "voxel_relief_m": 22,
       "micro_habitat_distinct": 0
     },
     "coherence": {