Tracks the window between SUBSCRIBE_OK and the first decoded audio
frame (typically 0.5-2s on a fresh subscription) so the user can
see audio is on its way rather than wonder why a "live" speaker
is silent.
Wire-up:
* AudioRoomUiState gains `connectingSpeakers: ImmutableSet<String>`,
a set-once-per-subscription field.
* VM enters the set on `slot.attach(...)` (right after SUBSCRIBE_OK
succeeds), exits on the first `onSpeakerActivity` callback (first
decoded packet), and clears on speaker removal. Set membership
survives subsequent silence — once a speaker has delivered a
frame, we know the pipeline works.
* ParticipantsGrid renders a small CircularProgressIndicator
overlaid on the avatar (sized smaller than the picture so the
user stays recognisable underneath).
Audience-side avatars don't get the overlay — they have no MoQ
subscription. Only on-stage speakers in the connectingSpeakers set
show it.
Claude
c3ff82913b