M6 DONE — torttube ships, Rick Astley plays fullscreen on the LibreELEC Pi

Live install verified end-to-end:
- SSH'd into kodi-host (LibreELEC, Kodi 20.3 Nexus, kernel aarch64
  / userspace armhf — that's why the static Rust sidecar runs but the
  PyInstaller yt-dlp binary couldn't)
- Dropped addon dir into /storage/.kodi/addons/
- systemctl restart kodi → Kodi rescans /storage/.kodi/addons/
- JSON-RPC Addons.SetAddonEnabled flipped enabled:false → true
- Player.Open with plugin URL → 7s yt-dlp resolve → VideoFullScreen.xml,
  fullscreen:true, currentwindow 12005, audio+video synced

Fixes that surfaced during the install:
- yt-dlp swap: PyInstaller aarch64 binary needs ld-linux-aarch64.so.1
  which LibreELEC doesn't ship. Switched to the universal Python zipapp
  (~3MB) which runs on /usr/bin/python3.11. build-addon-zip.sh updated.
- main.py now puts the addon's bin/ dir on PATH so the sidecar's
  Command::new('yt-dlp') call resolves to the bundled zipapp.
- Cosmetic fix: resolve.rs's classify_yt_dlp_error preserves the
  original error message (was downcasing it for keyword matching and
  then using the lowercased copy as the user-facing error).

Caveats logged for later:
- 360p ceiling (yt-dlp '-f best[ext=mp4]' picks itag 18; 720p
  progressive itag 22 is deprecated by YouTube; higher quality wants
  DASH manifest generation).
- ALSA sink: device 'sysdefault:CARD=vc4hdmi1' fails to open on this
  Pi but Kodi auto-falls-back to 'sysdefault' so audio works. Worth
  cleaning up in Kodi audio settings later.

MILESTONES + docs/install.md updated with the SSH + JSON-RPC alternate
install path.

MILESTONES.md

@@ -48,19 +48,27 @@
 - [ ] toast on skip + skip-counter in settings
 - [ ] category toggles in `settings.xml`
 
-## M6 — install + cross-compile [PARTIAL]
+## M6 — install + cross-compile [DONE]
 
 - [x] cross-compile sidecar for aarch64-musl static via throwaway
       `messense/rust-musl-cross:aarch64-musl` container. 6.2MB stripped
       static binary. Builds clean from `scripts/build-addon-zip.sh`.
-- [x] bundle yt-dlp's `yt-dlp_linux_aarch64` release binary for Tier 2/3
+- [x] bundle yt-dlp's universal Python zipapp (~3MB, not the PyInstaller
+      binary — that breaks on LibreELEC's armhf userspace because the
+      aarch64 dynamic loader doesn't exist; Python zipapp runs on
+      `/usr/bin/python3` which is always there).
 - [x] zip layout matches Kodi "install from zip" expectations
-- [x] addon.zip dropped at `smb://fileserver/downloads/torttube/` for Pi-side install
+- [x] addon.zip dropped at `smb://fileserver/downloads/torttube/`
 - [x] install + smoke recipe documented at `docs/install.md`
-- [ ] **install on the actual Pi** + verify the JSON-RPC `Player.Open`
-      smoke against `kodi-host` — needs the maintainer to either install via
-      Kodi UI or grant SSH to drop the zip directly
-- [ ] armv7 build for older Pis (deferred — the maintainer's TVs are all aarch64-capable)
+- [x] **installed on a LibreELEC Pi** (`kodi-host`) via SSH +
+      `systemctl restart kodi` + `Addons.SetAddonEnabled`
+- [x] **JSON-RPC `Player.Open` smoke verified** — Rick Astley played
+      end-to-end with audio + video synced (yt-dlp `-f best[ext=mp4]/best`
+      picked format 18, 360p H.264+AAC progressive)
+- [ ] armv7 build for older Pis (deferred — current TV is aarch64-capable
+      and my static sidecar runs on it even though userspace is armhf)
+- [ ] 720p+ playback (deferred to M5+) — itag 22 (720p progressive) is
+      deprecated by YouTube; higher quality needs DASH manifest generation
 
 ## Upstream PR work (parallel lane — every bug evaluated for "fix it upstream?")