MoQ: wait for the bot's broadcast before subscribing to it

[kixelated]

What

Both of the bot-side subscribers — the Watch.Broadcast catalog/audio watcher and the transcript stream — fired on this._reload.established, i.e. as soon as the connection came up. That assumes the bot is already publishing on the relay.

It isn't always. When the bot is started in response to the client's own announcement, its broadcast necessarily appears after ours. The relay resets a subscribe to a path nobody publishes, so both streams died with RESET_STREAM and neither retried, leaving the client connected but permanently silent:

subscribe start: id=0 broadcast=response/<id> track=transcript.json.z
subscribe start: id=1 broadcast=response/<id> track=catalog.json
announce:        broadcast=request/<id> active=true
subscribe error: id=1 ... track=catalog.json error=Received RESET_STREAM.
subscribe error: id=0 ... track=transcript.json.z error=Received RESET_STREAM.
announced:       broadcast=response/<id> active=true      <- bot arrives, too late

This doesn't affect the /start flow, where the server blocks on the bot's ready event before the browser ever connects.

How

Track the bot's broadcast announcement in a Signal and gate both subscribers on it. Watch.Broadcast already accepts an enabled signal, and the transcript effect re-runs when the signal flips — so each subscribes once the broadcast is really there, and re-subscribes if it goes away and comes back.

Testing

Verified against a bot that starts on demand, so its broadcast is guaranteed to appear after the client's. Before: both subscribes reset, no transcript, no audio. After: catalog fetch succeeds, transcript renders, sync[audio] shows the player consuming frames. tsc --noEmit and parcel build clean.

(written by Opus 5)

[kixelated]

Oh and I'm working on a better API for this, because yeah it's kind of gross. But I'll submit a separate PR for that.

[vipyne]

I pushed a commit, lmk if it works for you @kixelated. it's intended to be paired with pipecat PR #5158

[filipi87]

LGTM!