fix(capture): opaque mask overlay on Measure Zoom/Pan (iOS)

Alpha-punch of non-slab pixels left a mostly-transparent canvas inside the
CSS-transformed photo-layer. iOS Safari composites that blank — Ty's failure
(checkerboard + tape only, no wood) on LAN :10892. Paint opaque checkerboard
for non-slab like Mask; drop will-change. Unit-tested keep/cover counts.

frontend/src/capture/MeasurePanel.svelte

@@ -193,9 +193,8 @@
     </p>
 
     <!-- Same binding as Mask/Axis: oriented source + approved bitset.
-         PhotoStage tape mode punches non-slab alpha so the stage CSS checkerboard
-         shows through — no async createMaskedPreviewBlob swap (that path could
-         leave an empty/transparent canvas if encode hung or composited wrong). -->
+         PhotoStage paints opaque checkerboard on non-slab (iOS-safe under the
+         CSS pan/zoom/tilt transform — alpha punch left Zoom/Pan blank for Ty). -->
     <PhotoStage
       mode="tape"
       blob={state.source.blob}