Add background capture thread and live mode UI to DebugLayer

[Konstantysz]

Summary

• ProcessCurrentFrame() refactored into ProcessFrame(const Core::Frame &) — takes frame directly, no seek/grab inside
• New GrabCurrentReplayFrame() handles replay-mode seek+grab+process (with lastProcessedFrame guard)
• New StartCaptureThread() runs a background thread that calls source->Grab() in a loop and writes to a single-slot latestFrame (mutex-protected)
• OnUpdate() dispatches: live → take from latestFrame + ProcessFrame; replay → existing playback.Update() + GrabCurrentReplayFrame()
• Destructor stops capture thread (captureRunning = false + join()) before ImGui/GL teardown
• FPS counter updated once per second in OnUpdate() from totalFramesGrabbed (atomic)
• RenderPlaybackControls(): live mode shows ● LIVE X.X fps | N frames, no scrubber; replay mode unchanged
• RenderStatePanel(): live mode shows frame count, replay mode shows frame index / total

Test plan

• All 169 tests pass (no regressions)
• Replay mode: behavior identical to v0.1.0 (seek, scrubber, play/pause/step all work)
• Texture uploads remain on render thread — ProcessFrame() only called from OnUpdate() (render thread)
• Destructor: captureThread.join() called before GL teardown — no std::terminate

Closes #7