feat(simulation): activate background generation tier end-to-end (#968, D-206/D-225)

The D-206 queue (#924) and D-203 cache (#917) were built but never connected to
the running app — the whole background tier was inert. This closes the loop:
submit -> Rayon -> cascade -> completion -> drain -> cache.

- gen_queue.rs: GenWorkItem::AnalyzeBody now carries enqueuer-resolved inputs
  { body_id, heightmap_path, sea_level, body_seed: SeedChain } (D-225 boundary —
  run_work_item stays pure compute, no path/DB resolution). run_work_item runs
  the real cascade: load heightmap.png -> downsample to GRID_W×GRID_H working
  grid (D-202) -> run_layer1 -> BodyWorldState; load failure -> Failed.
  GenCompletion::BodyAnalyzed carries the computed BodyWorldState.
- cascade.rs: CascadeSnapshot::into_body_world_state() conversion.
- plugin.rs (new): GenerationPlugin registers GenerationQueue +
  BodyWorldStateCache and adds a PreInput drain system that inserts BodyAnalyzed
  states into the cache (off the Rayon workers — a cheap channel drain, never
  the ~45ms cascade). Wired into both the production and test app setups.

Tests run the real cascade on a tiny temp heightmap PNG (no committed fixture);
the plugin test proves the full submit->...->cache loop. The proxy (#969) is the
production submitter.

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

server/src/atlas/cascade.rs

@@ -17,6 +17,7 @@
 
 use std::path::Path;
 
+use crate::atlas::body_world_state::{BodyWorldState, RiverNetwork};
 use crate::atlas::heightmap::{self, BodyHeightmap, HeightmapLoadError};
 use crate::atlas::layer1::{self, Layer1Output};
 use crate::seed::SeedChain;
@@ -48,6 +49,29 @@ pub struct CascadeSnapshot {
     pub layer1: Option<Layer1Output>,
 }
 
+impl CascadeSnapshot {
+    /// Convert into a [`BodyWorldState`] for the D-203 cache (#968). Moves the
+    /// heightmap raster and the Layer-1 outputs in; `last_accessed` starts at 0
+    /// (the cache stamps it on read). A snapshot that stopped at Layer 0 yields
+    /// empty river/basin/attractor data.
+    pub fn into_body_world_state(self) -> BodyWorldState {
+        let (river_network, drainage_basins, attractors) = match self.layer1 {
+            Some(l1) => (l1.river_network, l1.drainage_basins, l1.attractors),
+            None => (RiverNetwork::default(), Vec::new(), Vec::new()),
+        };
+        BodyWorldState {
+            body_id: self.body_id,
+            heightmap: self.heightmap.data,
+            heightmap_width: self.heightmap.width,
+            heightmap_height: self.heightmap.height,
+            river_network,
+            drainage_basins,
+            attractors,
+            last_accessed: 0,
+        }
+    }
+}
+
 /// Run the cascade from a heightmap already in memory, up to `up_to`.
 ///
 /// Pure (no I/O); this is the testable core. `body_seed` is this body's