feat(validation): link run and fixture in regression summary

[hdamker]

What type of PR is this?

enhancement/feature

What this PR does / why we need it:

The regression runner's markdown summary now has Run and Fixture columns per branch. Run links to the camara-validation.yml Actions run whose findings were compared; Fixture links to the branch's .regression/regression-expected.yaml. Both fall back to - when the repo or run ID is not available. The runner is shared by the ReleaseTest canary sweep and the CommonalitiesTest sweep, so both summaries get the links.

• A reader can jump from a failing row straight to the run logs and the expected findings.
• DiffReport gains optional repo and run_id, so existing callers are unaffected.
• The example table in validation/docs/regression-testing.md is updated.

Which issue(s) this PR fixes:

N/A

Special notes for reviewers:

None.

Changelog input

 release-note

Additional documentation

docs

[rartych]

LGTM