Blur + remove background: cut out from the clean photo, clip the blur to it

[JamesmarkeyUK]

The bug

Blurring faces and then removing the background left an opaque blurred rectangle where the face was — the hair around it cut out cleanly, but the background inside the blur square survived.

removeBackground was running the segmentation model on img.file, which after a redaction is the baked blurred PNG. With no detail left inside the blur, the model classifies the entire blurred block as foreground, so nothing inside it gets cut away.

The fix

The blur is now modelled as a layer above the background edit, so the two compose in either order:

• removeBackground always segments the clean, un-blurred image (faceOriginal when a redaction is applied), so the matte is computed from real pixels.
• The redaction is re-baked over the finished cut-out, rendered from the opaque pre-removal photo and stencilled by the cut-out's alpha — renderRedacted's new mask argument, a destination-in composite. The blurred layer is deleted in exactly the same places the background is. Rendering from the opaque original (rather than blurring the transparent cut-out) keeps the silhouette crisp; resampling transparency directly would smear the alpha and fray the edge.
• bgOriginal / bgCutout now always hold clean snapshots, and faceOriginal holds the un-blurred image for whichever background state is current. Restore background and Remove blur stay available and undo independently, in any order. Restoring the background re-bakes the blur onto the full image rather than silently un-redacting the faces.
• Both re-bakes run before the new bitmap is swapped in (bakeFaceBlur is pure and touches no store state), so neither action ever shows an un-redacted frame.
• Face detection after a removal runs on the opaque original, not the subject-against-black cut-out.

Verification

npm run typecheck and npm run build pass.

The real renderRedacted was also driven in headless Chromium against a synthetic photo + cut-out, with a face box deliberately straddling the cut-out edge:

sample point	result
background area, outside face box	[0,0,0,0] — removed
inside face box, in the removed-background area	[0,0,0,0] — blur erased with the background
inside face box, inside subject	blurred, alpha 255
inside subject, outside face box	pixel-identical to original
no-mask path (blur only)	unchanged, opaque throughout

Round-trips checked by trace: blur → remove bg → restore bg → remove bg (cache hit) returns to a consistent state at each step, with no leaked or double-revoked object URLs.

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