Fix for dynacast error

[MaxHeimbrock]

Bug

When using dynacast: black video in calls (no frames ever arrive; the texture stays black) and the test failure (it
times out with zero frames — "no metadata" was really "no frames at all")

Proof: packet_trailer_test.rs fails when dynacast is on

Root cause

set_publishing_layers in client-sdk-rust~/livekit/src/room/track/local_video_track.rs:372 mishandles non-simulcast tracks (one encoding, empty rid):

1. It maps the empty rid to quality High (video_quality_for_rid_or_default("") → DEFAULT_VIDEO_QUALITY = High). But the SFU addresses layers by spatial index, so a single-layer track's only layer is addressed as Low. Reproduced in the test logs:
   dynacast: SFU quality update for TR_V...: subscribed_codecs="vp8:[Low=true, Medium=false, High=false]"
   dynacast: layers changed -> [(High)=off]
2. On top of that, a quality missing from the update is treated as "disable" (.unwrap_or(false)), so the mismatch turns the only encoding off. The publisher stops encoding entirely.

[github-actions]

Changeset

The following package versions will be affected by this PR:

Package	Bump
livekit	patch
livekit-ffi	patch