docs(planning): generate codebase map via gsd-map-codebase

.planning/codebase/INTEGRATIONS.md

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+# INTEGRATIONS
+
+## YouTube
+- **Mechanism**: Integrated natively through `yt-dlp` binary invocation via `subprocess`.
+- **Purpose**: Fetch the source guitar play-through videos asynchronously before frame extraction.
+- **Data Flow**: Downloads MP4 directly to the local filesystem (usually enforcing `res:720` for a balance between OCR fidelity and VRAM economy) before cv2 processing.
+
+## EasyOCR Model
+- **Mechanism**: Lazy-loaded `en` (English/Digits) detection network.
+- **Lifecycle**: Initialized on first call in `_get_ocr_reader()`. Downloads model weights if not locally cached in the Python environment.