fix(capture): show bg-removed cutout on Measure Zoom/Pan immediately

Tape-mode PhotoStage skipped mask checkerboard painting, so Zoom/Pan
depended on async createMaskedPreviewBlob — a hang/fail left the stage
without a cutout. Paint mask overlay in tape mode, mount PhotoStage on
source+bitset right away (swap to alpha PNG when ready), checkerboard
stage backdrop, and prefer HTML canvas for the preview encode.

frontend/src/capture/MeasurePanel.svelte

@@ -228,19 +228,21 @@
       {/if}
     </p>
 
-    {#if maskedPreview}
-      <PhotoStage
-        mode="tape"
-        blob={maskedPreview}
-        {tape}
-        {cropFill}
-        {gestureMode}
-        onTapeChange={onTapeChange}
-      />
-    {:else}
-      <div class="preview-loading" role="status">
-        {previewError || 'Preparing background-removed preview…'}
-      </div>
+    <!-- Cutout first: source + maskBitset paints checkerboard immediately (tape mode).
+         When createMaskedPreviewBlob finishes, swap to alpha PNG (same footprint). -->
+    <PhotoStage
+      mode="tape"
+      blob={maskedPreview ?? state.source.blob}
+      maskBitset={maskedPreview ? null : state.approvedMask.bitset}
+      {maskW}
+      {maskH}
+      {tape}
+      {cropFill}
+      {gestureMode}
+      onTapeChange={onTapeChange}
+    />
+    {#if previewError}
+      <p class="preview-err" role="status">{previewError}</p>
     {/if}
 
     <div class="controls">
@@ -449,7 +451,7 @@
   .mode-btn.active { background: #00e5ff; color: #002; }
   .mode-hint { margin: 0 0 10px; }
   .nudge { color: #e8a; font-size: 0.74rem; margin: 6px 0 0; line-height: 1.45; }
-  .preview-loading { min-height: 420px; border: 1px solid #333; border-radius: 8px; display: flex; align-items: center; justify-content: center; color: #999; text-align: center; padding: 16px; box-sizing: border-box; }
+  .preview-err { color: #ffb4a2; font-size: 0.85rem; margin: 8px 0 0; }
   .controls { margin-top: 16px; display: flex; flex-direction: column; gap: 14px; }
   .ctl-label { font-size: 0.78rem; color: #aaa; margin: 0 0 6px; }
   .ctl-label .sub { font-size: 0.7rem; color: #777; }

frontend/src/capture/PhotoStage.svelte

@@ -69,7 +69,9 @@
   };
 
   const paintOverlays = (ctx: CanvasRenderingContext2D) => {
-    if (!canvas || mode === 'tape') return;
+    if (!canvas) return;
+    // Mask checkerboard is valid in tape mode too (source-grid, moves with the photo).
+    // Bbox/axis arrow stay bbox-mode only — tape uses the fixed SVG fixture instead.
     if (maskBitset && maskW > 0 && maskH > 0) {
       const off = document.createElement('canvas');
       off.width = maskW;
@@ -91,6 +93,7 @@
       octx.putImageData(d, 0, 0);
       ctx.drawImage(off, 0, 0, canvas.width, canvas.height);
     }
+    if (mode === 'tape') return;
     if (bbox && bbox.corners.length === 4) {
       const c = bbox.corners;
       ctx.strokeStyle = '#00e5ff';
@@ -392,7 +395,16 @@
   .tape-stage { width: 100%; outline: none; }
   .tw {
     position: relative; width: 100%; aspect-ratio: 3 / 4;
-    background: #111; border: 1px solid #333; border-radius: 8px; overflow: hidden;
+    /* Checkerboard so bg-removed / transparent pixels read as cutout (same SoT as Mask). */
+    background-color: #eee;
+    background-image:
+      linear-gradient(45deg, #ccc 25%, transparent 25%),
+      linear-gradient(-45deg, #ccc 25%, transparent 25%),
+      linear-gradient(45deg, transparent 75%, #ccc 75%),
+      linear-gradient(-45deg, transparent 75%, #ccc 75%);
+    background-size: 16px 16px;
+    background-position: 0 0, 0 8px, 8px -8px, -8px 0;
+    border: 1px solid #333; border-radius: 8px; overflow: hidden;
     touch-action: none; user-select: none; cursor: grab;
   }
   .tw.dragging { cursor: grabbing; }

frontend/src/png/maskedPreview.ts

@@ -30,22 +30,30 @@ export async function createMaskedPreviewBlob(
   try {
     const width = bitmap.width;
     const height = bitmap.height;
-    const hasOffscreenCanvas = typeof OffscreenCanvas !== 'undefined';
-    const canvas: OffscreenCanvas | HTMLCanvasElement = hasOffscreenCanvas
-      ? new OffscreenCanvas(width, height)
-      : Object.assign(document.createElement('canvas'), { width, height });
+    if (bitset.length < width * height) {
+      throw new Error(`Mask bitset length ${bitset.length} < ${width * height}`);
+    }
+    // Prefer HTMLCanvasElement on the window thread — OffscreenCanvas.convertToBlob
+    // has hung / failed on some WebKit builds; worker code can still use Offscreen.
+    const useHtml = typeof document !== 'undefined';
+    const canvas: OffscreenCanvas | HTMLCanvasElement = useHtml
+      ? Object.assign(document.createElement('canvas'), { width, height })
+      : new OffscreenCanvas(width, height);
     const ctx = canvas.getContext('2d') as CanvasRenderingContext2D | OffscreenCanvasRenderingContext2D | null;
     if (!ctx) throw new Error('Could not create preview canvas');
     ctx.drawImage(bitmap, 0, 0);
     const image = ctx.getImageData(0, 0, width, height);
     image.data.set(applyMaskAlpha(image.data, bitset));
     ctx.putImageData(image, 0, 0);
-    if (hasOffscreenCanvas) {
-      return await (canvas as OffscreenCanvas).convertToBlob({ type: 'image/png' });
+    if (useHtml) {
+      return await new Promise<Blob>((resolve, reject) => {
+        (canvas as HTMLCanvasElement).toBlob(
+          (blob: Blob | null) => (blob ? resolve(blob) : reject(new Error('Could not encode preview'))),
+          'image/png',
+        );
+      });
     }
-    return await new Promise<Blob>((resolve, reject) => {
-      (canvas as HTMLCanvasElement).toBlob((blob: Blob | null) => (blob ? resolve(blob) : reject(new Error('Could not encode preview'))), 'image/png');
-    });
+    return await (canvas as OffscreenCanvas).convertToBlob({ type: 'image/png' });
   } finally {
     bitmap.close();
   }