fix(simulation): pre-build the true-keyed region cache (PR #199 review)

Hoshe finding 1: derive_all_districts passed a permanently-empty
region_cache, so every survey cell paid four on-the-fly baseline
derivations — and derive_district_profile's docstring (build the cache
before calling in a loop) was contradicted by its sole production
caller. The cache is now pre-built on the TRUE region keys the shared
core looks up (survey-cell centre metres -> containing district ->
region ±1 ring, BTreeSet-deduped), restoring the per-body build-once
cost model. Cache-hit == cache-miss byte-identity holds by D-227 purity
(build_region_profile and the miss branch share the same expressions) —
verified by the believability harness passing against the UNCHANGED
golden. Finding 2: the cascade.rs call-site comment and the stale
module header now state the pre-build truthfully.

Found in passing, filed as T-1186 (pre-existing, out of scope here):
region_centre_latitude_deg is pole-anchored while the derive core keys
equator-anchored signed regions — northern-hemisphere baselines clamp
to +90.

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

server/src/atlas/cascade.rs

@@ -306,8 +306,10 @@ pub fn run_cascade_from_heightmap(
                 // shared via scale::HEIGHTMAP_CELLS_PER_DISTRICT so plugin.rs
                 // converts CityPlacement pixel coords with the same constant.
                 // body_id is required for the D-243 §4 climate edge-fuzz warp
-                // domain separation — derive_all_districts builds the region
-                // cache internally.
+                // domain separation — derive_all_districts pre-builds the
+                // region-baseline cache internally on TRUE region keys (each
+                // survey cell's centre world metres → containing district →
+                // region ±1 ring; D-256(c) + PR #199 review).
                 //
                 // Layer1Output.survey_basin_dirs threads the true D8 thalweg
                 // direction into each DistrictProfile.basin_direction (T-1047)