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/body_world_state.rs

@@ -15,7 +15,9 @@ use serde::{Deserialize, Serialize};
 
 use crate::atlas::attractor_matching::CityPlacement;
 use crate::atlas::district_profile::{DistrictPos, DistrictProfile};
+use crate::atlas::region_profile::RegionProfile;
 use crate::atlas::road_graph::RoadGraph;
+use crate::atlas::scale::RegionPos;
 use crate::simulation::generator::{
     GeographicAttractor, QuarterId, QuarterWorldState, TerritorialStatus,
 };
@@ -100,6 +102,15 @@ pub struct BodyWorldState {
     /// `BTreeMap` keyed by `DistrictPos` for D-010 determinism.
     /// Empty until the DistrictProfile layer has run.
     pub districts: BTreeMap<DistrictPos, DistrictProfile>,
+    /// Per-region (~205 km) climate context — season/weather/temperature
+    /// baseline cells (D-243 §3, T-1113).
+    ///
+    /// Populated by the background cascade's Region layer; the covering grid
+    /// only (no blend-padding ring — see `cascade::LayerRegionOutput`).
+    /// `BTreeMap` keyed by `RegionPos` for D-010 determinism. Empty until the
+    /// Region layer has run. Footprint is trivial (~195×98 cells at the D-243
+    /// true-scale ceiling; a handful on today's working grids).
+    pub regions: BTreeMap<RegionPos, RegionProfile>,
     /// Last sim tick this entry was read. Used for LRU eviction.
     pub last_accessed: SimTick,
 }
@@ -224,6 +235,7 @@ mod tests {
             road_graph: RoadGraph::default(),
             quarters: BTreeMap::new(),
             districts: BTreeMap::new(),
+            regions: BTreeMap::new(),
             last_accessed: tick,
         }
     }