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;

src/BenchmarkLogger.h

@@ -28,6 +28,7 @@ struct BenchmarkFrame {
     int fps;
 
     // Detailed timing breakdown
+    float work_time_ms;       // Actual work time (frame_time - sleep_time)
     float grid_render_ms;
     float entity_render_ms;
     float python_time_ms;
@@ -172,6 +173,7 @@ public:
             file << "      \"timestamp_ms\": " << std::fixed << std::setprecision(3) << f.timestamp_ms << ",\n";
             file << "      \"frame_time_ms\": " << std::setprecision(3) << f.frame_time_ms << ",\n";
             file << "      \"fps\": " << f.fps << ",\n";
+            file << "      \"work_time_ms\": " << std::setprecision(3) << f.work_time_ms << ",\n";
             file << "      \"grid_render_ms\": " << std::setprecision(3) << f.grid_render_ms << ",\n";
             file << "      \"entity_render_ms\": " << std::setprecision(3) << f.entity_render_ms << ",\n";
             file << "      \"python_time_ms\": " << std::setprecision(3) << f.python_time_ms << ",\n";

src/GameEngine.cpp

@@ -291,6 +291,9 @@ void GameEngine::run()
             ImGui::SFML::Render(*window);
         }
 
+        // Record work time before display (which may block for vsync/framerate limit)
+        metrics.workTime = clock.getElapsedTime().asSeconds() * 1000.0f;
+
         // Display the frame
         if (headless) {
             headless_renderer->display();

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