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/simulation/generator.rs

@@ -986,7 +986,7 @@ pub struct DoorSpec {
 ///
 /// Integer-discriminant, D-010 compliant. `repr(u8)` pins values for
 /// serialisation stability; append-only.
-#[derive(Serialize, Deserialize, Clone, Debug, PartialEq, Eq, Default)]
+#[derive(Serialize, Deserialize, Clone, Copy, Debug, PartialEq, Eq, Default)]
 #[repr(u8)]
 pub enum MorphologyZone {
     /// Deep-water ocean — hub-and-spoke; perimeter access priority.