fix(simulation): wire region baseline into production district temp + rename latitude field (T-1078)

Addresses PR #167 review (Tyre T1, T3).

T1 — the region climate tier was built but no live path consulted it: both
derive_district_profile (cascade) and derive_district (on-demand) passed
region_baseline_c: None, so the D-243 §4 edge-fuzz blend was dead in production
and district temperature was still derived from scratch. Now both paths compute
region_baseline_at_district(...) and pass it through build_district_profile so
the two-phase modulation (D-243 §3) actually runs. The cascade path
(derive_all_districts) builds a BTreeMap<RegionPos, RegionProfile> covering all
district positions + their +/-1 neighbours once per body and looks up from it;
the on-demand path derives the four surrounding region baselines on the fly.
No production path passes None except genuinely airless bodies (no planet_class).

T3 — renamed BodyParams.district_latitude_deg -> latitude_deg (it is the region
centre latitude in the region fns, a call-site trap under the old name);
dual-scale doc; propagated to body_params_reader.rs and all literal sites.

Golden fixtures are unaffected (they construct districts directly via make_region,
bypassing build_district_profile) — derivation_harness 37/37 green, no re-pin.

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

server/src/atlas/cascade.rs

@@ -258,8 +258,15 @@ pub fn run_cascade_from_heightmap(
             // ~8 cells per district on a 128×64 working grid → ~80×32 = ~2 560 districts;
             // at full working resolution the budget is ~6 000/body (D-203).
             const CELLS_PER_REGION: usize = 8;
-            let districts =
-                district_profile::derive_all_districts(body_seed, params, &ta, CELLS_PER_REGION);
+            // body_id is required for the D-243 §4 climate edge-fuzz warp domain
+            // separation — derive_all_districts builds the region cache internally.
+            let districts = district_profile::derive_all_districts(
+                body_seed,
+                params,
+                &ta,
+                CELLS_PER_REGION,
+                &snapshot.body_id,
+            );
             snapshot.layer_district = Some(LayerDistrictOutput { districts });
         }