Fix output-diff publishing and post a new comment per run

[schani]

Changes

Support stacked PRs

The trusted publisher previously required every PR to target master. PR #3080 targets another branch, so its valid artifact was rejected and no report/comment was created.

When workflow_run supplies an exact PR number, validate that number plus the event's head repository, branch, and SHA without requiring a default-branch base. For fork events that omit the PR number, require exactly one open PR associated with the event's head owner and branch.

Post every current comparison as a new comment

Stop finding and patching an older marker-tagged output-diff comment. Every completed comparison for the PR's current head now creates a new comment, whether outputs differ or are clean. Stale comparisons still cannot comment.

Verification

• The exact metadata validation script passes against stacked PR fix(scala3): legalize JSON property names #3080 and fork PR C#: add --use-records option for C# 9+ record types #3071 artifacts.
• The comment step contains one unconditional comment POST, with no comment lookup or PATCH path.
• Workflow YAML parsing, Biome lint, and git diff --check pass.

[github-actions]

No generated-output differences

✅ This PR does not change generated outputs.