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/atlas/body_params_reader.rs

@@ -1,9 +1,9 @@
 //! Pre-dispatch reader for body physical parameters (T-1032, D-239 §1).
 //!
-//! Reads the columns needed to construct a [`BodyParams`] for the RegionProfile
+//! Reads the columns needed to construct a [`BodyParams`] for the DistrictProfile
 //! carrier layer from `systems.db` at `AnalyzeBody` enqueue time.  This keeps
 //! the Rayon work item DB-free while supplying the climate/tectonic inputs
-//! required by `derive_all_regions`.
+//! required by `derive_all_districts`.
 //!
 //! **D-240:** orbit/star fields (`orbital_period_days`, `axial_tilt_deg`,
 //! `spectral_class`, `star_type`) are non-canonical placeholder data — they are
@@ -23,8 +23,8 @@
 //!
 //! `tectonic_activity` is **not** in the current schema; `BodyParams.tectonic_activity`
 //! is left `None` so the derivation falls back to `planet_class` as documented
-//! on the struct.  The per-region fields `region_latitude_deg` and `elevation_km`
-//! are set by `derive_all_regions` / `derive_region_profile`, not here; they
+//! on the struct.  The per-district fields `district_latitude_deg` and `elevation_km`
+//! are set by `derive_all_districts` / `derive_district_profile`, not here; they
 //! remain at their struct defaults (0.0) from this reader.
 //!
 //! Read-only `systems.db` access follows the same pattern as
@@ -37,7 +37,7 @@ use std::sync::{Arc, Mutex};
 use rusqlite::{Connection, OpenFlags};
 use thiserror::Error;
 
-use crate::atlas::region_profile::BodyParams;
+use crate::atlas::district_profile::BodyParams;
 
 // ---------------------------------------------------------------------------
 // Error type
@@ -119,9 +119,9 @@ impl BodyParamsReader {
                 planet_class,
                 // tectonic_activity not in schema — leave None.
                 tectonic_activity: None,
-                // Per-region fields are set by derive_all_regions / derive_region_profile,
+                // Per-district fields are set by derive_all_districts / derive_district_profile,
                 // not at the body level. Leave at struct defaults (0.0).
-                region_latitude_deg: 0.0,
+                district_latitude_deg: 0.0,
                 elevation_km: 0.0,
                 body_radius_km,
             }),
@@ -185,7 +185,13 @@ mod tests {
         conn.execute(
             "INSERT INTO bodies (body_id, hydrosphere, atmosphere, planet_class, body_radius_km)
              VALUES (?1, ?2, ?3, ?4, ?5)",
-            rusqlite::params![body_id, hydrosphere, atmosphere, planet_class, body_radius_km],
+            rusqlite::params![
+                body_id,
+                hydrosphere,
+                atmosphere,
+                planet_class,
+                body_radius_km
+            ],
         )
         .expect("insert body");
 
@@ -209,8 +215,8 @@ mod tests {
         assert_eq!(params.atmosphere.as_deref(), Some("breathable"));
         assert_eq!(params.planet_class.as_deref(), Some("temperate"));
         assert_eq!(params.body_radius_km, Some(6371.0));
-        // Per-region fields always start at 0.0 from the reader.
-        assert_eq!(params.region_latitude_deg, 0.0);
+        // Per-district fields always start at 0.0 from the reader.
+        assert_eq!(params.district_latitude_deg, 0.0);
         assert_eq!(params.elevation_km, 0.0);
         // tectonic_activity not in schema → None.
         assert!(params.tectonic_activity.is_none());
@@ -238,7 +244,13 @@ mod tests {
 
     #[test]
     fn read_is_deterministic() {
-        let db = make_test_db("GJ4d", Some("ice"), Some("thin"), Some("frozen"), Some(3389.5));
+        let db = make_test_db(
+            "GJ4d",
+            Some("ice"),
+            Some("thin"),
+            Some("frozen"),
+            Some(3389.5),
+        );
         let reader = BodyParamsReader::open(&db).expect("open");
         let p1 = reader.read_body_params("GJ4d").expect("first read");
         let p2 = reader.read_body_params("GJ4d").expect("second read");