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/AiCommandProcessor.cs

@@ -83,6 +83,13 @@ public sealed class AiCommandProcessor
             .Set(new Transform(command.Position, command.Rotation, command.Scale))
             .Set(mesh);
 
+        if (command.Physics)
+        {
+            var maxScale = MathF.Max(MathF.Max(command.Scale.X, command.Scale.Y), command.Scale.Z);
+            var rb = RigidBody.DynamicBox(new Vector3(maxScale * 0.5f), mass: maxScale * 2f);
+            entity.Set(rb);
+        }
+
         return AiCommandResult.Ok($"Spawned entity '{command.Name}' with model '{command.ModelPath}' (id {(ulong)entity.Id}).");
     }
 

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;
 }

src/Engine.AI/Mcp/EngineMcpTools.cs

@@ -24,7 +24,8 @@ public sealed class EngineMcpTools
         string modelPath,
         [Description("Optional position as [x, y, z] (default: 0,0,0)")] IReadOnlyList<double>? position = null,
         [Description("Optional rotation as [x, y, z, w] quaternion (default: identity)")] IReadOnlyList<double>? rotation = null,
-        [Description("Optional scale as [x, y, z] (default: 1,1,1)")] IReadOnlyList<double>? scale = null)
+        [Description("Optional scale as [x, y, z] (default: 1,1,1)")] IReadOnlyList<double>? scale = null,
+        [Description("Optional: enable physics (default: false)")] bool physics = false)
     {
         var cmd = new SpawnModelCommand
         {
@@ -32,7 +33,8 @@ public sealed class EngineMcpTools
             ModelPath = modelPath,
             Position = ToVector3(position, Vector3.Zero),
             Rotation = ToQuaternion(rotation, Quaternion.Identity),
-            Scale = ToVector3(scale, Vector3.One)
+            Scale = ToVector3(scale, Vector3.One),
+            Physics = physics
         };
 
         return EnqueueAndReturnMessage(cmd);