Fix asymmetric album art crop -- was showing more top-left, losing bottom/right

Root-caused with the calibration tool's crosshair marker: at rest and
near-rest (zoom breathing cycle's endpoints, where Scale is momentarily
pinned to a constant value), animation.Origin has no effect at all --
confirmed empirically, varying origin_x/origin_y produced byte-identical
output whenever Scale doesn't actually change frame to frame, regardless
of the scale value itself. With nothing for Origin to pivot, the crop
fell back to render.Box's own default handling of an oversized
Transformation child, which isn't centered: the crosshair sat several
pixels down-and-right of true center, meaning the art was effectively
showing more of its own top-left and cropping away a disproportionate
chunk of the bottom-right -- exactly "losing ~10% on bottom and right,
when it's not zoomed".

Fixed with a *constant* Translate (same value at every keyframe, not
animated) -- found empirically via the calibration tool (the (tx, ty)
that centered the crosshair at rest also held up at peak zoom, checked
against both). This isn't the mistake an earlier, abandoned attempt
made: that version's Translate *varied* over the animation fighting
Origin's own pivot, and interacted badly with Scale's continuous
interpolation (the historical "periodic pop" bug, per pixlet's own
source on Translate's per-frame rounding). A constant value never
changes frame to frame, so there's nothing for it to round-mismatch
against -- confirmed smooth across a full cycle.

Scaled by `scale` (box_crop_bias = -2 * scale) to stay correct at 2x.
dev/calibration_test.star updated to match, per its own sync requirement
with the real _zoom_transformation.

Verified: crosshair now dead-center at rest (calibration tool), 0
black-corner hits for jitter+bands in isolation at worst-case settings
(strong zoom, random position) at both 1x and 2x, and 0 black/void
corners across a full zoom cycle at all four ZOOM_RANDOM_SPOTS values
with strong zoom.

dev/calibration_test.star

@@ -32,6 +32,19 @@ clumsily approximating), confirmed via this calibration tool with
 genuinely different (non-default) origin values -- they do change the
 output, despite an earlier, flawed test (origin set to its own default
 and compared against itself) suggesting `origin` had no effect at all.
+
+Later: `origin` turned out to only have any effect while Scale is
+actually changing frame to frame (confirmed with this tool: varying
+origin_x/origin_y produced identical output for a Scale pinned to a
+constant value at every keyframe). At the two instants this animation's
+own Scale is momentarily constant (percentage 0.0 and 1.0, "rest"), the
+crop falls back to render.Box's own default handling of an oversized
+Transformation child, which isn't centered -- the crosshair below sat
+several pixels toward the bottom-right of center. Fixed with BOX_CROP_BIAS,
+a *constant* Translate (same value at every keyframe, found via this
+tool's crosshair) -- not the same mistake as the first attempt above,
+since a constant value never changes frame to frame, so there's nothing
+for it to round-mismatch against Scale's interpolation.
 """
 
 load("render.star", "render")
@@ -41,6 +54,7 @@ load("calibration.png", CALIB = "file")
 ART_SIZE = 36
 ART_SLOT = 32
 ART_ZOOM_DURATION = 200
+BOX_CROP_BIAS = -2  # see the docstring above; keep in sync with box_crop_bias in the real _zoom_transformation
 
 def _art_widget(art_bytes, zoom_scale, origin_x, origin_y):
     art = render.Image(src = art_bytes, width = ART_SIZE, height = ART_SIZE)
@@ -50,9 +64,9 @@ def _art_widget(art_bytes, zoom_scale, origin_x, origin_y):
         duration = ART_ZOOM_DURATION,
         origin = animation.Origin(origin_x, origin_y),
         keyframes = [
-            animation.Keyframe(percentage = 0.0, transforms = [animation.Scale(1.0, 1.0)]),
-            animation.Keyframe(percentage = 0.5, transforms = [animation.Scale(top_scale, top_scale)], curve = "ease_in_out"),
-            animation.Keyframe(percentage = 1.0, transforms = [animation.Scale(1.0, 1.0)], curve = "ease_in_out"),
+            animation.Keyframe(percentage = 0.0, transforms = [animation.Scale(1.0, 1.0), animation.Translate(BOX_CROP_BIAS, BOX_CROP_BIAS)]),
+            animation.Keyframe(percentage = 0.5, transforms = [animation.Scale(top_scale, top_scale), animation.Translate(BOX_CROP_BIAS, BOX_CROP_BIAS)], curve = "ease_in_out"),
+            animation.Keyframe(percentage = 1.0, transforms = [animation.Scale(1.0, 1.0), animation.Translate(BOX_CROP_BIAS, BOX_CROP_BIAS)], curve = "ease_in_out"),
         ],
     )
     return render.Box(width = ART_SLOT, height = ART_SLOT, child = zoom)