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/UPSTREAM.md

@@ -0,0 +1,38 @@
+# Tracking upstream YTSage
+
+SageTube keeps the full git history of [YTSage](https://github.com/oop7/YTSage)
+and the internal `ytsage` package name so upstream improvements can be merged.
+
+## Syncing
+
+```bash
+git fetch upstream            # upstream = https://github.com/oop7/YTSage.git
+git merge upstream/main
+```
+
+## Expected conflict zones
+
+Most SageTube features live in **new files** and merge cleanly. Conflicts are
+likely only in:
+
+| File | Reason |
+|---|---|
+| `ytsage/gui/ytsage_gui_main.py` | tab-shell edit in `init_ui` (`_setup_main_tabs`), closeEvent hook |
+| `ytsage/gui/ytsage_gui_analysis.py` | delegates subprocess execution to `YtdlpClient` |
+| `ytsage/utils/ytsage_constants.py` | SageTube data dirs, PATH prepend for managed binaries |
+| `ytsage/utils/ytsage_config_manager.py` | atomic save, defaults merge, player/feed/advanced keys |
+| `pyproject.toml` | name, version, deps (python-mpv), URLs |
+| `ytsage/languages/en.json` | added sections: player, cards, main_tabs, search, watch, browse, feed |
+
+Bug fixes made in this fork (see git log between the merge commit and the
+first feature commit) may also conflict if upstream fixes them differently —
+prefer whichever version is stricter about verification and thread safety.
+
+## SageTube-only modules (never conflict)
+
+- `ytsage/core/ytsage_client.py` — shared yt-dlp JSON invocation layer
+- `ytsage/core/ytsage_mpv.py` — libmpv probe
+- `ytsage/gui/ytsage_gui_player.py` — embedded mpv player
+- `ytsage/gui/ytsage_gui_watch.py` / `_search.py` / `_browse.py` / `_feed.py` — tabs
+- `ytsage/gui/ytsage_gui_router.py` / `_cards.py` — navigation + video cards
+- `ytsage/utils/ytsage_library_manager.py` — subscriptions/history/queue DB