Add fork documentation and rewrite README

- README: watch-first feature overview, install + libmpv requirement,
  fork attribution.
- docs/UPSTREAM.md: upstream merge procedure, expected conflict zones,
  list of SageTube-only modules.
- docs/BUILDING.md: per-OS libmpv install, managed-binary locations,
  playback reliability notes, warning that inherited GitHub release
  workflows download appimagetool/ffmpeg without checksum verification.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/BUILDING.md

@@ -0,0 +1,48 @@
+# Building & running SageTube
+
+## Requirements
+
+- Python >= 3.10
+- **libmpv** (system library; `python-mpv` binds to it at runtime)
+  - Arch: `sudo pacman -S mpv`
+  - Debian/Ubuntu: `sudo apt install libmpv2`
+  - Fedora: `sudo dnf install mpv-libs`
+  - macOS: `brew install mpv`
+  - Windows: place `libmpv-2.dll` next to the executable (builds from
+    https://github.com/shinchiro/mpv-winbuild-cmake/releases), or install mpv
+    and add it to PATH
+- ffmpeg (for download merging; the app can auto-install it)
+
+Without libmpv the app still runs — the Watch tab shows an install hint and
+search/browse/download work normally.
+
+## Run from source
+
+```bash
+python -m venv .venv && source .venv/bin/activate
+pip install -e .
+sagetube
+```
+
+On first start the app downloads its own SHA256-verified `yt-dlp` binary and
+the Deno runtime (yt-dlp's JS challenge solver) into the SageTube data dir:
+
+- Linux: `~/.local/share/SageTube/bin/`
+- macOS: `~/Library/Application Support/SageTube/bin/`
+- Windows: `%LOCALAPPDATA%\SageTube\bin\`
+
+## Notes on YouTube playback reliability
+
+Playback resolves streams through mpv's `ytdl_hook` using the managed yt-dlp.
+YouTube's CDN intermittently serves stalled streams to non-browser clients;
+the player retries automatically (twice, after 25 s of no playback). Keeping
+yt-dlp updated (built-in updater, stable or nightly channel) is the long-term
+fix channel for extractor breakage.
+
+## Release workflows
+
+The `.github/workflows/` files are inherited from upstream YTSage and target
+GitHub runners; they do not run on Gitea. If you set up Gitea Actions, note
+that the upstream Linux/Windows workflows download appimagetool and a bundled
+ffmpeg **without checksum verification** — add hash checks before reusing
+them for release artifacts.