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/voxel.rs

@@ -3118,7 +3118,7 @@ mod tests {
         ];
         for zone in &zones {
             let district = DistrictProfile {
-                morphology_zone: zone.clone(),
+                morphology_zone: *zone,
                 tectonic_class: TectonicClass::Volcanic,
                 glaciation_grade: GlaciationGrade::Moderate,
                 slope_q: 50,
@@ -3153,7 +3153,7 @@ mod tests {
         ];
         for (zone, expected_terrain) in cases {
             let district = DistrictProfile {
-                morphology_zone: zone.clone(),
+                morphology_zone: *zone,
                 tectonic_class: TectonicClass::Volcanic,
                 glaciation_grade: GlaciationGrade::Moderate,
                 slope_q: 50,