feat: optional physics in spawn_model MCP command

- SpawnModelCommand: added Physics bool property (default false)
- AiCommandProcessor.SpawnModel: when Physics=true, adds RigidBody.DynamicBox
  with half-extent = max(scale) * 0.5, mass = max(scale) * 2
- EngineMcpTools.SpawnModel: added 'physics' parameter (default false)
- Physics is optional — spawn without physics by default, enable when needed
- Physics bodies are initialized by the existing per-frame IsInitialized check

src/Engine.AI/Commands/SpawnModelCommand.cs

@@ -12,4 +12,5 @@ public sealed record SpawnModelCommand : AiCommand
     public Vector3 Position { get; init; } = Vector3.Zero;
     public Quaternion Rotation { get; init; } = Quaternion.Identity;
     public Vector3 Scale { get; init; } = Vector3.One;
+    public bool Physics { get; init; } = false;
 }