firmware: live-update last_paint_event_us_low so /state g2g reflects real frame age

Previous Phase 4 implementation only published last_paint_event_us_low
into the windowed-stats snapshot at every 30-frame roll (~1.5s at
20fps). Combined with the script's 5s /state poll cadence, the
"freshest frame age" we could compute was up to ~6.5s stale before
any actual lag — meaningless as a perf signal.

Update s_stats.last_paint_event_us_low under portMUX on every
painted v1 frame. The other window stats (recv/dec/paint/idle
min/avg/max + fps/MBps) keep their roll cadence because they
genuinely need a full window to compute; this single u32 just gets
overwritten with the most recent value each paint.

portENTER_CRITICAL on the ESP32-P4 is ~half a microsecond per side
— at 20fps that's 20µs/s of overhead, immeasurable next to the
30-40ms recv per frame.

Expected: g2g during a saturated stream drops from the previous
2-6s reading to single-digit hundreds of ms or lower, reflecting
the actual emit→display lag.

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