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/GameEngine.h

@@ -34,6 +34,7 @@ struct ProfilingMetrics {
     float fovOverlayTime = 0.0f;     // Time spent rendering FOV overlays (ms)
     float pythonScriptTime = 0.0f;   // Time spent in Python callbacks (ms)
     float animationTime = 0.0f;      // Time spent updating animations (ms)
+    float workTime = 0.0f;           // Total work time before display/sleep (ms)
 
     // Grid-specific metrics
     int gridCellsRendered = 0;       // Number of grid cells drawn this frame