g2g semantics now per-frame, drop sharp mozjpeg for ~4x faster snapshot

Two follow-up fixes informed by the first-round field data:

#1 — g2g measures display age, not time-since-wake
Previously eventUsLow was stamped once at wake event and reused for
the whole stream session. /state's last_paint_event_us_low echoed
back the wake-time anchor, so g2g = "elapsed time since user wake."
That metric is true but not actually informative — it just grows
linearly with stream duration.

What's actually useful for the "is the panel showing current
reality" question is the AGE of the currently-displayed frame.
Stamp event_us_low per emitted frame at Date.now()*1000 (low 32
bits) inside the stream demux loop. Now last_paint_event_us_low
reflects the most recent painted frame's emit time, and
g2g = (script_now_us_low - last_paint_event_us_low) = display
staleness in ms.

#2 — sharp first-paint dropped from ~1800ms to ~800ms
Previous commit (phase 5) turned on sharp's mozjpeg encoder for
slightly tighter file size at the same JPEG quality. Field
measurement showed it costs ~1.5s of CPU per snapshot vs
libjpeg-turbo's ~400ms — a ~4× regression on the metric we care
most about (event → first-paint). The file-size win is ~5%; on
800x480 panel output that's not perceptible.

Drop mozjpeg: true. Quality settings (quality=100, chroma 4:4:4 at
jpegQuality ≤ 2) stay the same, so visual output is unchanged.
First-paint comes back down to where it should be.

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