- Add `--plugin-action <id>/<name>` CLI flag, dispatched via runPluginAction
- Add ctx.ui.openWindow for plugin-owned BrowserWindows with sandboxed preload
- Add ctx.dataDir (`plugin-data/<id>/`), ctx.actions.run, ctx.mpv.ensureRunning
- Emit mpv end-file events to plugins with reason, entry id, and error
- Run Windows .cmd/.bat shims through cmd.exe in exec, killing the tree on abort
- Update plugin docs, usage, architecture notes, and e2e coverage
