Fix #219: Add threading support with mcrfpy.lock() context manager

Enables background Python threads to safely modify UI objects by
synchronizing with the render loop at frame boundaries.

Implementation:
- FrameLock class provides mutex/condvar synchronization
- GIL released during window.display() allowing background threads to run
- Safe window opens between frames for synchronized UI updates
- mcrfpy.lock() context manager blocks until safe window, then executes
- Main thread detection: lock() is a no-op when called from callbacks
  or script initialization (already synchronized)

Usage:
    import threading
    import mcrfpy

    def background_worker():
        with mcrfpy.lock():  # Blocks until safe
            player.x = new_x  # Safe to modify UI

    threading.Thread(target=background_worker).start()

The lock works transparently from any context - background threads get
actual synchronization, main thread calls (callbacks, init) get no-op.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/PyLock.h

@@ -0,0 +1,25 @@
+#pragma once
+// #219 - Thread synchronization context manager for mcrfpy.lock()
+
+#include <Python.h>
+
+// Forward declaration
+class GameEngine;
+
+// PyLockContext - the context manager object returned by mcrfpy.lock()
+typedef struct {
+    PyObject_HEAD
+    bool acquired;  // Track if we've acquired the lock
+} PyLockContextObject;
+
+// The type object for the context manager
+extern PyTypeObject PyLockContextType;
+
+// Module initialization function - adds lock() function to module
+namespace PyLock {
+    // Create the lock() function that returns a context manager
+    PyObject* lock(PyObject* self, PyObject* args);
+
+    // Initialize the type (call PyType_Ready)
+    int init();
+}