feat(simulation): add entity-entity collision

validate_movement now checks both terrain walkability AND tile
occupancy. Builds a spatial index of occupied tiles from stationary
entities, then resolves movers in order — first valid claim wins.

Same spatial pattern needed for D-026 simulation tiers (30-80 active
NPCs) and future pathfinding occupied-tile awareness.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/tests/movement.rs

@@ -52,3 +52,31 @@ fn movement_validated_within_app() {
     assert!(app.world().get::<MoveIntent>(mover).is_none());
     assert!(app.world().get::<MoveIntent>(blocked).is_none());
 }
+
+#[test]
+fn entity_collision_blocks_movement() {
+    let mut app = App::new();
+    app.add_plugins(SimulationPlugin);
+    app.insert_resource(WalkabilityMap::new(10, 10, 1));
+
+    // Stationary entity at (5,4)
+    app.world_mut().spawn(TilePosition::new(5, 4, 0));
+
+    // Mover tries to move into occupied tile
+    let mover = app
+        .world_mut()
+        .spawn((
+            TilePosition::new(5, 5, 0),
+            MoveIntent {
+                target: TilePosition::new(5, 4, 0),
+            },
+        ))
+        .id();
+
+    app.update();
+
+    assert_eq!(
+        *app.world().get::<TilePosition>(mover).unwrap(),
+        TilePosition::new(5, 5, 0)
+    );
+}