fix(audit): disposition verified GT exporter differences

[developer0hye]

Summary

• add a strict, machine-readable manifest for verified reference-exporter differences
• match layout visibility dispositions by exact page, payload, and occurrence
• match render dispositions by the manifest's exact stable cluster-ID set
• require hashed full-page native evidence, structured source/reference/native provenance, and a stable verification-comment URL
• keep unrelated layout findings and render clusters gated on open issues
• document the manifest, CLI flag, and PR-body fields

Related issue

Related: #1519, #1418, #1421

Testing

• python3 -m unittest discover -s scripts/tests (374 passed)
• python3 -m compileall -q scripts
• git diff --check
• rehearsed the contract against PPTX page-9 title rule is 11.5pt short and 3.6pt high #1418's real page-9 visibility mismatch and all 20 current strict render-cluster IDs; both reports passed with exact reference-difference selectors
• exported the original PPTX ordinary shape picture fill is omitted from slide 1 #1220 PPTX with Microsoft PowerPoint 16.112.3 on macOS 26.6.2 and inspected 300-DPI GT/output/native title and slide-number crops; native PowerPoint matches office2pdf for both scoped differences

Visual impact

• No rendered PDF change
• Rendered PDF change or visual evidence added
• Reason: This changes audit validation, tests, and documentation only; conversion and raster output are unchanged, and no issue evidence asset is added.

Checklist

• Commits include a Signed-off-by line
• PR scope contains one root cause
• Remaining converter or harness deviations each reference an open issue

[developer0hye]

Merge-order gate: keep this PR open until #1520 has merged. The current #1520 strict report uses open issue #1519 as its disposition; merging this contract first is technically safe because issue closure is manual, but preserving the explicit order avoids invalidating that live evidence. After #1520 merges, this PR can merge and #1519 can close only after the new contract is adopted on fresh page-9 evidence.