feat: Step 6 MCP server bridge for AI agents

- Add ModelContextProtocol + ModelContextProtocol.AspNetCore 1.4.0 to Engine.AI.
- Expose AI commands as MCP tools: spawn_model, set_transform, delete_entity, list_entities.
- Add AiCommandQueue to marshal commands from the MCP server thread to the main thread.
- Start in-process HTTP MCP server in Program.cs (Debug/Release only; excluded in ReleaseAOT).
- Add --mcp-port CLI argument to configure the MCP server port.
- Fix Flecs.NET package conditions to include ReleaseAOT config.

src/Engine.AI/AiCommandQueue.cs

@@ -0,0 +1,63 @@
+using System.Collections.Concurrent;
+using System.Text.Json;
+using Engine.AI.Commands;
+
+namespace Engine.AI;
+
+/// <summary>
+/// Thread-safe queue that marshals AI commands from the MCP server thread
+/// to the main engine thread where the Flecs world is touched.
+/// </summary>
+public sealed class AiCommandQueue
+{
+    private readonly ConcurrentQueue<(string commandJson, TaskCompletionSource<AiCommandResult> tcs)> _queue = new();
+    private readonly AiCommandProcessor _processor;
+    private readonly JsonSerializerOptions _jsonOptions;
+
+    public AiCommandQueue(AiCommandProcessor processor)
+    {
+        _processor = processor;
+        _jsonOptions = processor.JsonOptions;
+    }
+
+    /// <summary>
+    /// Enqueue a command object. The returned task completes on the main thread
+    /// when the command has been processed.
+    /// </summary>
+    public Task<AiCommandResult> EnqueueAsync(AiCommand command)
+    {
+        var json = JsonSerializer.Serialize<AiCommand>(command, _jsonOptions);
+        return EnqueueJsonAsync(json);
+    }
+
+    /// <summary>
+    /// Enqueue a raw JSON command string.
+    /// </summary>
+    public Task<AiCommandResult> EnqueueJsonAsync(string commandJson)
+    {
+        var tcs = new TaskCompletionSource<AiCommandResult>(TaskCreationOptions.RunContinuationsAsynchronously);
+        _queue.Enqueue((commandJson, tcs));
+        return tcs.Task;
+    }
+
+    /// <summary>
+    /// Process all pending commands. Must be called on the main engine thread.
+    /// Returns the number of processed commands.
+    /// </summary>
+    public int ProcessPending()
+    {
+        int processed = 0;
+        while (_queue.TryDequeue(out var item))
+        {
+            var result = _processor.Process(item.commandJson);
+            item.tcs.TrySetResult(result);
+            processed++;
+        }
+        return processed;
+    }
+
+    /// <summary>
+    /// Number of commands waiting to be processed.
+    /// </summary>
+    public int PendingCount => _queue.Count;
+}