feat(simulation): wire region climate layer into the cascade + proxy (T-1113, D-243)

CascadeLayer::Region appended per the enum's append-only Ord rule (depends only on seed/body_params/heightmap dims — documented); BodyWorldState.regions BTreeMap per the districts precedent; RegionGridLayer dense row-major all-integer encoding (season/weather repr(u8) discriminants, mean_temp deci-degC i16 with i16::MIN airless sentinel, moisture_q u8); build_region_grid mirrors build_district_grid; protocol.gd region_grid passthrough (visual overlay deliberately out of scope); wire fixtures regenerated via make fixtures. atlas:: suite 507 green.

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

server/src/atlas/gen_queue.rs

@@ -407,13 +407,14 @@ fn run_work_item(item: &GenWorkItem) -> GenCompletion {
                 } else {
                     hm
                 };
-                // Run the full cascade through RoadGraph (Layer 2, T-1038), the
-                // terminal layer. It subsumes Settlement, DistrictProfile (T-1023),
-                // and all prior layers. DistrictProfile derivation still gates on
-                // body_params internally (skipped when absent — e.g. a body with no
-                // params row), but the road graph needs no body params, so it runs
-                // for every analyzed body.
-                let up_to = CascadeLayer::RoadGraph;
+                // Run the full cascade through Region (T-1113), the terminal
+                // layer. It subsumes RoadGraph (T-1038), Settlement,
+                // DistrictProfile (T-1023), and all prior layers.
+                // DistrictProfile and Region derivation both gate on
+                // body_params internally (skipped when absent — e.g. a body
+                // with no params row), but the road graph needs no body
+                // params, so it runs for every analyzed body.
+                let up_to = CascadeLayer::Region;
                 let snapshot = run_cascade_from_heightmap(
                     *body_seed,
                     working,