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.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();