M6 — cross-compile aarch64-musl + addon.zip + install docs

scripts/build-addon-zip.sh runs the whole pipeline from a host with ssh build-host:
- one-shot messense/rust-musl-cross:aarch64-musl container builds the
  sidecar static (6.2MB stripped). Doesn't mutate the build container.
- fetches yt-dlp_linux_aarch64 from the upstream release page so Tier 2
  + Tier 3 work on the Pi (LibreELEC ships no Python YouTube tools)
- packages everything into plugin.video.torttube.zip with the Kodi
  install-from-zip layout
- drops the zip at /srv/downloads/torttube/ on the file share

Cargo.toml swaps rustypipe to default-features=false +
rustls-tls-webpki-roots so the cross-compile is openssl-free.

addon.xml drops the unused script.module.requests requirement — main.py
only uses Python stdlib + Kodi's own modules.

docs/install.md walks the Kodi UI flow + a smoke curl that fires
Player.Open via JSON-RPC. Pi-side smoke is pending install on
kodi-host.

addon/plugin.video.torttube/addon.xml

@@ -5,7 +5,6 @@
        provider-name="Sulkta">
   <requires>
     <import addon="xbmc.python" version="3.0.0"/>
-    <import addon="script.module.requests" version="2.22.0"/>
     <import addon="inputstream.adaptive" version="2.0.0" optional="true"/>
   </requires>
   <extension point="xbmc.python.pluginsource" library="main.py">