rp2040js runs at ~50% real time, so a TFT frame burst (fillRect sky +
fillRect floor + many drawFastVLine for walls + HUD) often takes longer
than 16 ms to drain through the SPI pipeline. Painting on every rAF
captured mid-burst snapshots that the next sky fill immediately
clobbered, so the canvas only ever showed the last few pixels written
before each tick — most visibly the raycaster examples rendering 2-3
wall columns instead of 160.
Strategy: each SPI pixel write resets a 16 ms idle timer. We paint only
after that period of silence (a real frame boundary), with a 100 ms
hard cap so continuous-write sketches still update.
Also adds test/pico_doom_demo/raycaster-perf.mjs — a puppeteer-based
profiler that reports CPU step rate, SPI throughput, per-pixel cost,
and paint rate. Run with the dev backend + frontend up:
node test/pico_doom_demo/raycaster-perf.mjs
After the fix the Doom raycaster paints at the sketch's natural 10 FPS
with full frames (was 29 fps of mid-burst snapshots).
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