Spatial audio for AudioStream and VideoPlayer

Protocol adds `spatial`, `spatial_min_distance`, `spatial_max_distance` to both PBAudioStream (4-6) and PBVideoPlayer (7-9). Not in our vendored protos; `AudioStream::position()` is hardcoded to 0 (`crates/av/src/lib.rs:435`) so playback is always global, and video audio is likewise non-spatial. We already spatialize AudioSource, so the emitter machinery exists.

Unity ships both, including live updates (`MediaStream/MultiMediaPlayer.cs`, `UpdateMediaPlayerSystem.cs`) — live parity gap. Protocol refs: #323/#412.