feat(simulation): atlas layer-proxy handler — cache hit/miss -> queue (#969, D-225)

handle_atlas_request: cache hit -> serialize Layer1Output, reply Ready; miss ->
resolve the source heightmap (BodySourceResolver) + enqueue an Immediate
AnalyzeBody on the #968 background queue, reply Pending (client re-requests; the
drain system populates the cache so a later request hits); unknown/no-terrain ->
NotFound. Adds the AtlasLayerRequest/Response/Status wire types (+ Serialize on
CascadeLayer). Pure handler; the bridge routing is the proxy's other half.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

server/src/atlas/cascade.rs

@@ -17,6 +17,8 @@
 
 use std::path::Path;
 
+use serde::{Deserialize, Serialize};
+
 use crate::atlas::body_world_state::{BodyWorldState, RiverNetwork};
 use crate::atlas::heightmap::{self, BodyHeightmap, HeightmapLoadError};
 use crate::atlas::layer1::{self, Layer1Output};
@@ -25,7 +27,7 @@ use crate::seed::SeedChain;
 /// Cascade layers in execution order (D-200). [`run_cascade`] runs every layer
 /// up to and including the requested one. Append new layers as they are built;
 /// the `Ord` derive relies on declaration order, so only ever append.
-#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Serialize, Deserialize)]
 pub enum CascadeLayer {
     /// Layer 0 — load the pre-baked 16-bit `heightmap.png` (D-202).
     Heightmap,