docs: initialize project

.planning/REQUIREMENTS.md

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+# Requirements
+
+## Objective
+Rebuild `youtube_tab_to_pdf.py` Computer Vision pipeline from the ground up to achieve 100% continuous measure extraction without skips or overwrites, primarily resolving the "discontinuous measure numbers" issue in YouTube guitar tabs.
+
+## Scenarios
+- **SCN-1: The Playhead Problem.** Videos often contain a vertical red/blue bar tracking the current play position. This cursor moves across the screen and disrupts image matching.
+- **SCN-2: The Repeating Chorus Problem.** In music, measure 50 might look identical to measure 10. The system must not confuse current frame context with a previous frame 40 measures ago and overwrite the timeline.
+- **SCN-3: Sub-optimal measure bars.** Videos compress measure bar lines making them hard to detect accurately, so the system must rely on chronological time-shift tracking.
+
+## Acceptance Criteria
+- [ ] `test_pipeline.py` passes for all 3 sample URLs showing no missing sections between start and end.
+- [ ] Output panoramas/chunks are continuously ordered from start to finish without jumping back to an earlier identical part of the song.
+- [ ] The moving playhead indicator is fully removed in the final PDF chunks.
+- [ ] CV Logic is moved out of the main wrapper into a concise, easily testable `video_cv_tracker.py`.
+
+## Out of Scope
+- Building a UI/Frontend.
+- Changing `yt-dlp` download logic.
+- Supporting arbitrary instruments (Piano/Drums) other than 6-string Guitar Tabs.