fix(action): reduce false-positive stale flags and orphaned new-images in manifest workflows

[danadajian]

Summary

Two related refinements to the manifest workflows, both aimed at eliminating misleading review noise:

1. Skip flagging open PRs whose overlapping changeset values match the merged changeset.

• When a PR merges, manifest-merge flags every open PR whose stored changeset overlaps the merged changeset on any screenshot path, failing its Visual Regression status with "please rebase". The overlap check compared paths only, so an open PR whose hash for a shared path is identical to the value that just merged — meaning it fully agrees with the new baseline and could not clobber it — was still flagged.
• Observed in a PR: merging a PR flagged 12 open PRs as stale, at least one of which carried the exact same hash for its overlapping path as the merged changeset and posed no conflict risk.
• The overlap check now also requires the two changesets to disagree on the path's value (treating matching deletions as agreement, and a deletion vs. an update as a conflict), so only PRs that would actually overwrite the new baseline get flagged.

2. Delete uploaded new-images that fall outside the final review set during manifest-compare.

• manifest-generate uploads a new.png for every screenshot whose hash differs from the base branch's manifest at generate time, but compare may later exclude some of those paths from review — visually identical to their base image (hash noise from nondeterministic rendering), owned by main, or stale from a prior run. The leftover files stayed under new-images/{sha}/, and since the Comparadise UI lists everything under that prefix, they appeared as spurious "new" screenshots with no base to compare against.
• Observed in in PR: a PR's Comparadise page showed a "new" screenshot for a test that compare had explicitly dropped as visually identical and correctly excluded from the changeset and PR comment.
• After compare settles on the final review set, any uploaded image files for paths outside it are now deleted (in match, main-owns-only, and classified outcomes; conflict outcomes bail early and are left untouched), so the UI only ever shows what actually needs review.

Both changes come with tests, and the committed action bundle is rebuilt.

🤖 Generated with Claude Code

[eg-oss-ci]

🎉 This PR is included in version 1.46.12 🎉

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