[playbridgeapp]

Phase 3 of Watch Together (docs/design-watch-together.md): the desktop as a screen, a remote and a host. Stacked on #533; retarget to main once #533 merges.

Desktop

• Watch Together screen in the sidebar: start a party (shared relay or your own), join with a link, or show a code and QR so a phone can add this computer.
• Screen: "Watch on this computer" (on unless a TV is the chosen screen) plays the party video in the desktop player, loaded paused, outside history and resume points. Play, pause and seek from keys, the player, media keys, the extension or a phone remote go to the party; speed nudges bypass the speed ladder. Hiding the window and "Still watching?" don't pause a party. Playing something else or stopping the player turns this computer into a remote.
• Remote and host: play, pause, seek, copy the invite, add the connected PlayBridge TV (it becomes this computer's own and, for the host, its delegate) or a TV by code, lock, who can change the video, requests, sign-in sharing.
• Paired phone handoff: the desktop advertises watch_together_v1; the Rust receiver passes an authenticated sender's watch_together_join to the app, which joins and answers watch_together_joined with its member ID and keys.
• Extension: a video sent during a party plays in the party (direct links; synthetic playlists are refused).
• Dart binding for pb_together_* in packages/playbridge_cast_core_dart; desktop builds (script and PR CI) include the together feature.

Engine (all platforms)

• The room pauses once no device shows the video, and a video waits for a screen before it starts. Found while testing: with every player stopped, the party clock kept running.
• Live streams (every screen loaded them with no length) share only playing or paused: no seeks or speed nudges, since live positions differ per device.
  • The room announces a live party; the phone and desktop party screens hide seeking, and seeks are refused.
  • A buffering device doesn't pause a live party (that would put everyone behind live).
  • Play after a pause jumps every device to its live point (go_live: ExoPlayer default position, mpv seek 100 absolute-percent; new seekToLive on the TV renderer interface).
  • TV fix: when it isn't told in time whether a video is live, it uses the length it already knows instead of assuming live.
• Phone, TV and desktop native libraries rebuilt.

Tests

• Desktop: 449 Flutter tests, including 10 new Watch Together tests.
• Dart binding: 5 new tests. Kotlin: shared, phone and TV unit tests and builds.
• Rust: 27 + 17 together tests (3 new simulations, live covered for buffering, seeks and go-live), 51 FFI tests, receiver handoff test; fmt and clippy clean.
• On this Mac: a desktop-hosted party with the Samsung joining worked, and with the new engine the party paused once both players were stopped. Live streams are covered by simulations only. Windows and Linux share the code but are untested.