Rework transcription commit logic

At the core of the STT, there's a loop which uses Whisper to convert
audio into a transcript. As you say something, whisper sees growing
fragments of your sentence:

t0: "Hell"
t1: "Hello"
t2: "Hello, world!"

So we need some algorithm which takes these fragments and
accumulates them into an ever-growing transcript.

Previously I did this with fuzzy string matching. I'd find the region
where the two transcripts overlap and edit the two together to produce a
longer transcript. The big problem is that if there's no overlap, it's
not clear whether whisper radically changed its mind as to what was
said, or whether the user paused for a long time before saying
something new. So I'd have to reset the growing transcript.

Now I get the timestamps from Whisper and wait for it to give me the
same 3 transcripts for the last utterance. Once the transcript
stabilizes like this, I commit the text. This enables a temporally
stable, ever-growing transcript that's also quite accurate.

To prevent a latency regression, I also introduce the notion of "preview
text", which is a preview of an utterance that has not yet stabilized.
These previews do not contribute to the ever-growing transcript, but do
get fed through the rest of the app, so they show up in-game / in OBS.
Once they eventually stabilize, they get committed to the ever-growing
transcript.

This change is lightly tested!

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