Add warning when starting benchmark in headless mode

The benchmark API captures per-frame data from the game loop, which is
bypassed when using step()-based simulation control. This warning
informs users to use Python's time module for headless performance
measurement instead.

Also adds test_headless_benchmark.py which verifies:
- step() and screenshot() don't produce benchmark frames
- Wall-clock timing for headless operations
- Complex scene throughput measurement

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/McRFPy_API.cpp

@@ -1355,6 +1355,14 @@ PyObject* McRFPy_API::_setDevConsole(PyObject* self, PyObject* args) {
 // Benchmark logging implementation (#104)
 PyObject* McRFPy_API::_startBenchmark(PyObject* self, PyObject* args) {
     try {
+        // Warn if in headless mode - benchmark frames are only recorded by the game loop
+        if (game && game->isHeadless()) {
+            PyErr_WarnEx(PyExc_UserWarning,
+                "Benchmark started in headless mode. Note: step() and screenshot() do not "
+                "record benchmark frames. The benchmark API captures per-frame data from the "
+                "game loop, which is bypassed when using step()-based simulation control. "
+                "For headless performance measurement, use Python's time module instead.", 1);
+        }
         g_benchmarkLogger.start();
         Py_RETURN_NONE;
     } catch (const std::runtime_error& e) {