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/tests/gen_fixtures.rs

@@ -624,6 +624,7 @@ fn generate_atlas_layer_response_fixtures() {
         district_grid: None,
         road_graph: Some(road_graph),
         settlements: Some(settlements),
+        region_grid: None,
     };
     write_fixture(
         "atlas_response_ready",
@@ -637,6 +638,7 @@ fn generate_atlas_layer_response_fixtures() {
         district_grid: None,
         road_graph: None,
         settlements: None,
+        region_grid: None,
     };
     write_fixture(
         "atlas_response_pending",
@@ -650,6 +652,7 @@ fn generate_atlas_layer_response_fixtures() {
         district_grid: None,
         road_graph: None,
         settlements: None,
+        region_grid: None,
     };
     write_fixture(
         "atlas_response_not_found",