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.

1 files changed, 12 insertions, 1 deletions

diff --git a/main/stream_server.c b/main/stream_server.c
index 1321f22..206c513 100644
--- a/main/stream_server.c
+++ b/main/stream_server.c
@@ -239,7 +239,18 @@ static void handle_client(int fd, const char *peer)
         viewport_state_unlock();
 
         last_painted_seq = seq;
-        if (event_us_low != 0) last_event_us_low = event_us_low;
+        if (event_us_low != 0) {
+            last_event_us_low = event_us_low;
+            // Live update: /state needs this fresh on every painted
+            // frame so the script's g2g reflects the actual age of
+            // the currently-displayed frame, not the age at last
+            // window roll (which can be up to ~1.5s stale). The
+            // other window stats stay at roll cadence — they
+            // genuinely need a full window to compute.
+            portENTER_CRITICAL(&s_stats_mux);
+            s_stats.last_paint_event_us_low = event_us_low;
+            portEXIT_CRITICAL(&s_stats_mux);
+        }
         jpeg_decoder_unlock();
         state_machine_frame_painted();
         frames_decoded++;