feat: Add work_time_ms to benchmark logging for load analysis

Track actual work time separately from frame time to determine
system load percentage:
- work_time_ms: Time spent doing actual work before display()
- sleep_time = frame_time_ms - work_time_ms

This allows calculating load percentage:
  load% = (work_time / frame_time) * 100

Example at 60fps with light load:
- frame_time: 16.67ms, work_time: 2ms
- load: 12%, sleep: 14.67ms

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

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

src/BenchmarkLogger.cpp

@@ -16,6 +16,7 @@ void BenchmarkLogger::recordFrame(const ProfilingMetrics& metrics) {
     frame.frame_time_ms = metrics.frameTime;
     frame.fps = metrics.fps;
 
+    frame.work_time_ms = metrics.workTime;
     frame.grid_render_ms = metrics.gridRenderTime;
     frame.entity_render_ms = metrics.entityRenderTime;
     frame.python_time_ms = metrics.pythonScriptTime;