Add a calibration test image/script for visually checking zoom cropping

dev/calibration.png is a synthetic grid (concentric color rings, corner
markers, a crosshair) and dev/calibration_test.star runs it through the
same zoom logic as the real _art_widget, so cropping can be checked
visually without depending on what a live station happens to be playing.
This is what was used to find and verify the centered-zoom fix.

README.md

@@ -128,6 +128,21 @@ pixlet render subwave_nowplaying.star station_url=https://your-station.example.c
 pixlet serve subwave_nowplaying.star
 ```
 
+`dev/calibration_test.star` renders a synthetic test grid (`dev/calibration.png`
+— concentric color rings, corner markers, a crosshair) through the exact same
+zoom logic as the real `_art_widget`, to visually check cropping without
+depending on what a live station happens to be playing:
+
+```
+pixlet render dev/calibration_test.star -o calib.gif
+```
+
+At rest you should see a thin, *even* sliver of magenta on all four edges;
+as it zooms in, each corner marker should get clipped by the same amount.
+If one side loses noticeably more than the others, the zoom isn't
+centered. Keep its copy of the zoom math in sync with `_art_widget` if that
+ever changes.
+
 ## Deploy
 
 Upload `subwave_nowplaying.star` directly through your Tronbyt server's