CVE-2025-64503: libcupsfilters, backport upstream fix (no version bump)

[jasonodoom]

⚠️ EXPERIMENTAL source-backport — NOT a version bump

This PR authors the upstream fix as an in-tree patch against the current nixpkgs version (no version bump). An agent generated it; a cross-family reviewer verified it transcribes the upstream commit, and a cert proved the bundled reproducer goes red→green. A human maintainer must still confirm sufficiency before merging:

• This applies the COMPLETE upstream fix, not one of several required commits.
• The backported hunk matches the upstream fix commit(s):
  • OpenPrinting/libcupsfilters@fd01543
• Reviewed the cert evidence (reproducer red→green) for CVE-2025-64503.

Opened as a draft deliberately — mark Ready-for-review only after the boxes above are checked.

---

Evidence: CVE-2025-64503 (human-review-required) trace bundle 7d39d3572dd1af31

• Rekor entry: https://rekor.sigstore.dev/api/v1/log/entries?logIndex=2727406735
• Workflow identity: https://github.com/Ad-Astra-Computing/trace/.github/workflows/steward.yml@refs/heads/main
• OIDC issuer: https://token.actions.githubusercontent.com
• Cert run: https://github.com/Ad-Astra-Computing/trace/actions/runs/33975628773
• nixpkgs rev: f13ec9852ca452df38492a0c30ad60b49efc1e50

Verify locally

gh run download <run-id> --name steward-smoke-<bundle> --dir ./artifact
trace bundle verify ./artifact  # bundle_id=7d39d3572dd1af31

Generated by trace.

[jasonodoom]

Comparison against the upstream fix

Transcription lane (this PR's payload). The added and removed code lines of the embedded patch (cupsfilters/pdftoraster.cxx) are identical to the upstream fix commit libcupsfilters fd01543f, normalized for whitespace. The cross-family Reviewer attested this; the check here was recomputed independently against the bundle's upstream-fix.patch. The Linux cert (trace run 33975628773) built libcupsfilters at the pinned revision with and without the patch: the unpatched arm fired the discriminator, the patched arm was clean.

Blind-authoring lane (separate measurement, not this payload). When the same CVE was given to claude-opus-4-8 with only the vulnerability description and the pre-fix source, never the upstream patch, the graded result against the human fix was WRONG: it changed indentation and whitespace only and added no bounds check on the page dimensions, so the integer overflow and undersized buffer remained. Report: trace data/reports/authoring-accuracy-vs-human-2026-09-04.json.

The gap between the two lanes is the case for the gate. A blind fix that changes nothing would have passed a build but not the Reviewer's line-level comparison against the upstream fix.