dummy cv inference pipeline with mock ui overlay

[HenryWashuHe]

Summary

• Adds CV inference pipeline: frame source -> model runner -> post-processor -> screen-space overlay
• Mock runner for editor testing, ORT runner for on-device ONNX inference
• Debug HUD showing FPS, detection count, inference time
• Includes test frames and smoke ONNX model in StreamingAssets
• Updated .gitignore to exclude .DS_Store and .vscode/

Note

This is a mockup/prototype — mock model returns hardcoded detections. Real preprocessing (resize/normalize/CHW) is still TODO.

Test plan

• Open project in Unity, enter Play mode — should see green bounding box overlay from mock detections
• Debug HUD should show FPS, detection count, and "mock" model status