feat(simulation): sprint 9 gauntlet — test infrastructure and first 3 rooms

Add Gauntlet test world with 3 rooms (Inventory Warehouse, Occlusion
Corridor, Pause Chamber) + Central Hub, room constants module, room
reset trigger mechanism, Layer 3 subprocess integration test, golden
file comparison engine and test suite, and content runtime validation.

Tickets: #482, #484, #485, #487, #488, #489, #490

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

server/src/knowledge/registry.rs

@@ -69,6 +69,19 @@ impl EntityRegistry {
         }
     }
 
+    /// Advance the next StableId counter to `target`.
+    /// Used to reserve StableId ranges for entities not yet spawned
+    /// (e.g., Gauntlet rooms built in later sprints).
+    /// Panics if `target` is less than the current next_id.
+    pub fn reserve_up_to(&mut self, target: u64) {
+        assert!(
+            target >= self.next_id,
+            "cannot reserve backwards: next_id={}, target={}",
+            self.next_id, target
+        );
+        self.next_id = target;
+    }
+
     /// Number of registered entities.
     pub fn len(&self) -> usize {
         self.by_entity.len()
@@ -229,6 +242,30 @@ mod tests {
         );
     }
 
+    #[test]
+    fn reserve_up_to_advances_counter() {
+        let mut world = World::new();
+        let mut registry = EntityRegistry::new(0);
+
+        let e1 = world.spawn_empty().id();
+        let id1 = registry.register(e1);
+        assert_eq!(id1, StableId(0));
+
+        // Reserve through 5 (skip IDs 1-4)
+        registry.reserve_up_to(5);
+
+        let e2 = world.spawn_empty().id();
+        let id2 = registry.register(e2);
+        assert_eq!(id2, StableId(5), "next ID after reserve should be 5");
+    }
+
+    #[test]
+    #[should_panic(expected = "cannot reserve backwards")]
+    fn reserve_up_to_panics_on_backwards() {
+        let mut registry = EntityRegistry::new(10);
+        registry.reserve_up_to(5);
+    }
+
     #[test]
     fn unregister_unknown_entity_is_noop() {
         // #469: Unregistering an entity that was never registered must not panic.