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

dev/calibration_test.star

@@ -0,0 +1,50 @@
+"""
+Dev tool, not part of the deployed app: renders dev/calibration.png through
+the exact same zoom logic as _art_widget in subwave_nowplaying.star, so you
+can visually check how much margin gets cropped at rest and at full zoom,
+and whether the crop is even on all sides.
+
+calibration.png is a 360x360 grid: concentric 10%-wide color rings (magenta
+outermost, then yellow, then cyan) plus four corner markers (red TL, green
+TR, blue BL, white BR) and a center crosshair. 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/corner loses
+noticeably more than the others, the zoom isn't centered.
+
+Usage:
+    pixlet render dev/calibration_test.star -o calib.gif
+    pixlet render dev/calibration_test.star zoom_scale=1.0 -o calib_rest.gif  # rest only
+
+Keep ART_SIZE/ART_SLOT/the zoom math below in sync with the real
+_art_widget if that ever changes.
+"""
+
+load("render.star", "render")
+load("animation.star", "animation")
+load("calibration.png", CALIB = "file")
+
+ART_SIZE = 36
+ART_SLOT = 32
+ART_ZOOM_DURATION = 200
+
+def _art_widget(art_bytes, zoom_scale):
+    art = render.Image(src = art_bytes, width = ART_SIZE, height = ART_SIZE)
+    top_scale = max(zoom_scale, 1.0)
+    shift = (1.0 - top_scale) * (ART_SIZE / 2.0)
+    zoom = animation.Transformation(
+        child = art,
+        duration = ART_ZOOM_DURATION,
+        keyframes = [
+            animation.Keyframe(percentage = 0.0, transforms = [animation.Scale(1.0, 1.0), animation.Translate(0, 0)]),
+            animation.Keyframe(percentage = 0.5, transforms = [animation.Scale(top_scale, top_scale), animation.Translate(shift, shift)], curve = "ease_in_out"),
+            animation.Keyframe(percentage = 1.0, transforms = [animation.Scale(1.0, 1.0), animation.Translate(0, 0)], curve = "ease_in_out"),
+        ],
+    )
+    return render.Box(width = ART_SLOT, height = ART_SLOT, child = zoom)
+
+def main(config):
+    zoom_scale = float(config.get("zoom_scale", "1.4"))
+    return render.Root(child = render.Row(children = [
+        _art_widget(CALIB.readall(), zoom_scale),
+        render.Box(width = 32, height = 32, color = "#111111"),
+    ]))