Treat a zero-width feather transition as a hard edge so exact-radius pixels remain unchanged, including at zero opacity. Add actual image regressions for all spot modes, image corners and two clone output sizes.
Validation: both new regressions failed on the original implementation and pass here. Formatting, workspace Clippy, HEIF and a generated-photo app smoke test pass. The unfiltered workspace gate remains blocked by the existing all-masks overlay test tracked by #1599; later parity, layers, assets and WASM stages are pending. This is a source checkpoint, not a green release candidate.
Contributor: @eliets2
eliets2
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