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>