refactor(simulation): rename carrier RegionProfile->DistrictProfile + re-scale to the D-243 ladder (T-1077)

Phase 2 of the D-243 re-scale. The ~1km carrier is renamed and re-scaled to
the 2km district, and every consumer reads scale from the canonical scale.rs
instead of a local literal.

- region_profile.rs -> district_profile.rs; RegionProfile -> DistrictProfile,
  RegionPos -> DistrictPos (re-exported from scale::DistrictPos), derive_*_regions
  -> derive_*_districts, across all 10 consumers + the derivation harness.
- chunk_context now references scale:: for the carrier cell: district 1024m -> 2048m,
  shift 4 -> 5 (CHUNK_DISTRICT_SHIFT). This caught a real latent bug: the seed-district
  index used a literal '>> 4' while the anchor used the constant, so same-district
  chunks could derive different anchors — now both use scale::CHUNK_DISTRICT_SHIFT.
- derivation_harness scale literals ('>> 4', '* 1024', '0..16') converted to
  scale:: constants (the same drift the canonical source eliminates). Believability
  tests pass at 2km; the voxel determinism golden re-pinned (anchors moved with the
  re-scale — deterministic, intended).

Full suite + clippy --all-targets -D warnings green. The district DERIVATION still
tiles the heightmap at 8 cells (the old coarse path); replacing it with on-demand
heightmap interpolation + detail-scatter is phase 3.

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

server/src/main.rs

@@ -216,7 +216,7 @@ fn main() {
         ),
     }
 
-    // Body physical params reader for RegionProfile carrier layer (T-1032, D-239 §1, D-240):
+    // Body physical params reader for DistrictProfile carrier layer (T-1032, D-239 §1, D-240):
     // reads hydrosphere / atmosphere / planet_class on a cache miss so the Rayon
     // cascade work item stays DB-free (D-225 pattern).
     // D-240: orbit/star fields are non-canonical and are no longer read.
@@ -229,7 +229,7 @@ fn main() {
             );
         }
         Err(e) => tracing::warn!(
-            "Body params reader unavailable ({}). RegionProfile layer will be skipped.",
+            "Body params reader unavailable ({}). DistrictProfile layer will be skipped.",
             e
         ),
     }