feat(slice2): add PhysicsEngine.IsLandblockLoaded — readiness gate §3.4

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/AcDream.Core/Physics/PhysicsEngine.cs

@@ -31,6 +31,15 @@ public sealed class PhysicsEngine
     /// <summary>Number of registered landblocks (diagnostic).</summary>
     public int LandblockCount => _landblocks.Count;
 
+    /// <summary>
+    /// Returns true iff a landblock with this id has been registered via
+    /// <see cref="AddLandblock"/>. Because <see cref="AddLandblock"/> registers
+    /// terrain + collision cells + portals atomically, ContainsKey implies the
+    /// collision mesh is present and resolvable.
+    /// </summary>
+    public bool IsLandblockLoaded(uint landblockId) =>
+        _landblocks.ContainsKey(landblockId);
+
     /// <summary>
     /// Cell-based spatial index for static object collision.
     /// Populated during landblock streaming; queried by the Transition system.