feat(simulation): serve the coarse district/morphology grid through the Atlas proxy (T-1046)

Server half of wiring the derived tier into production. The D-225 proxy
(AtlasLayerResponse) carried only Layer1Output, so the Atlas could not show the
DistrictProfile tier that the cascade derives. Adds DistrictGridLayer (cols/rows
+ row-major MorphologyZone discriminants + elev_q for relief shading) and
build_district_grid(), populated on a cache hit. Because the response is
rmp_serde msgpack end-to-end, the new field reaches the client automatically.

This is the planetary-scale coarse grid (the Atlas map view); the on-demand 2km
derive_district (T-1077) is for in-world Phase 5, not the map. MorphologyZone
gains Copy (fieldless repr(u8) enum; additive).

Next: client decode (protocol.gd) + the D-226 generation overlay in the Atlas
viewer.

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

server/src/atlas/plugin.rs

@@ -79,6 +79,7 @@ fn serve_atlas_requests(
                 body_id: req.body_id.clone(),
                 status: AtlasLayerStatus::Error("no body source resolver".to_string()),
                 layer1: None,
+                district_grid: None,
             },
         };
         responses.0.push(resp);