feat: Add benchmark logging system for performance analysis (closes #104)

Add Python API for capturing performance data to JSON files:
- mcrfpy.start_benchmark() - start capturing frame data
- mcrfpy.end_benchmark() - stop and return filename
- mcrfpy.log_benchmark(msg) - add log message to current frame

The benchmark system captures per-frame data including:
- Frame timing (frame_time_ms, fps, timestamp)
- Detailed timing breakdown (grid_render, entity_render, python, animation, fov)
- Draw call and element counts
- User log messages attached to frames

Output JSON format supports analysis tools and includes:
- Benchmark metadata (PID, timestamps, duration, total frames)
- Full frame-by-frame metrics array

Also refactors ProfilingMetrics from nested GameEngine struct to
top-level struct for easier forward declaration.

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

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

src/BenchmarkLogger.cpp

@@ -0,0 +1,37 @@
+#include "BenchmarkLogger.h"
+#include "GameEngine.h"
+
+// Global benchmark logger instance
+BenchmarkLogger g_benchmarkLogger;
+
+void BenchmarkLogger::recordFrame(const ProfilingMetrics& metrics) {
+    if (!running) return;
+
+    auto now = std::chrono::high_resolution_clock::now();
+    double timestamp_ms = std::chrono::duration<double, std::milli>(now - start_time).count();
+
+    BenchmarkFrame frame;
+    frame.frame_number = ++frame_counter;
+    frame.timestamp_ms = timestamp_ms;
+    frame.frame_time_ms = metrics.frameTime;
+    frame.fps = metrics.fps;
+
+    frame.grid_render_ms = metrics.gridRenderTime;
+    frame.entity_render_ms = metrics.entityRenderTime;
+    frame.python_time_ms = metrics.pythonScriptTime;
+    frame.animation_time_ms = metrics.animationTime;
+    frame.fov_overlay_ms = metrics.fovOverlayTime;
+
+    frame.draw_calls = metrics.drawCalls;
+    frame.ui_elements = metrics.uiElements;
+    frame.visible_elements = metrics.visibleElements;
+    frame.grid_cells_rendered = metrics.gridCellsRendered;
+    frame.entities_rendered = metrics.entitiesRendered;
+    frame.total_entities = metrics.totalEntities;
+
+    // Move pending logs to this frame
+    frame.logs = std::move(pending_logs);
+    pending_logs.clear();
+
+    frames.push_back(std::move(frame));
+}