Fix the periodic zoom pop at its root: disable Translate's pixel rounding

Root-caused the "pop" from the Scale+Translate centered zoom (reverted a
few commits back) to pixlet's own source: render/animation/translate.go
rounds a Translate's x/y to the nearest whole pixel every frame by
default (rounding = "round"), while animation/scale.go never rounds
Scale. A continuously-interpolating Scale combined with a rounded
Translate holds still for several frames, then jumps a whole pixel at
once -- a real, confirmed pixlet behavior, not a flaw in this app's
formula.

Transformation accepts a `rounding` parameter; passing "none" disables
the snapping. Confirmed via frame-by-frame pixel-delta analysis: the
per-frame change is now even throughout the animation (consistently
~900 AE out of 1024 possible, every frame) instead of long near-zero
stretches punctuated by sudden full-frame jumps.

Re-enables the centered zoom (Scale + Translate) that was reverted for
jerkiness, now smooth. ZOOM_LEVELS["strong"] stays capped at 1.45 --
that black-border ceiling is unrelated to rounding and still applies.
Verified against the live station: no pop, no black border at any zoom
level, payload ~370KB (up from ~220-320KB, since rounding="none" means
WebP can dedupe fewer near-identical frames) but still well under the
460KB device limit.

dev/calibration_test.star

@@ -1,7 +1,8 @@
 """
 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.
+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
@@ -14,17 +15,20 @@ Usage:
 Keep ART_SIZE/ART_SLOT/the zoom math below in sync with the real
 _art_widget if that ever changes.
 
-Note: a centered (Scale + Translate) version of this zoom was tried and
-visually fixed the top-left crop bias below, but combining Scale with
-Translate causes pixlet to periodically re-rasterize the whole image with
-a visible "pop" -- confirmed with both WebP and GIF output, independent of
-zoom amplitude, so it's pixlet's animation engine, not app-level math. This
-tool (and the real app) stick to Scale alone -- a real but minor crop-bias
-cosmetic issue, instead of the pop a centered zoom introduces. If you want
-to re-investigate centering, add animation.Translate back in here first
-and compare frame-to-frame pixel deltas (see git history around the
-"Center the album art zoom" / "Revert jerky zoom fix" commits) before
-touching the real app.
+History, if this ever needs revisiting: a Scale+Translate centered zoom
+was tried, then reverted after it produced a periodic rendering "pop"
+(confirmed with both WebP and GIF output, independent of zoom amplitude).
+Root cause, found in pixlet's own source
+(render/animation/translate.go): a Translate's x/y are rounded to the
+nearest whole pixel every frame by default (`rounding = "round"`), while
+Scale is never rounded -- so a continuously-interpolating Scale combined
+with a rounded Translate holds still for several frames, then jumps a
+whole pixel at once. Passing `rounding = "none"` on the Transformation
+(see below) disables that snapping and fixed it -- confirmed via
+frame-by-frame pixel-delta analysis (even per-frame change throughout,
+instead of long still stretches punctuated by sudden jumps). If a future
+change reintroduces jerkiness, check whether `rounding = "none"` is still
+present before suspecting the shift formula.
 """
 
 load("render.star", "render")
@@ -38,13 +42,15 @@ 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,
+        rounding = "none",
         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(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)