Add pypdf PNG predictor advisory references

[cookesan]

Adds pypdf PNG predictor advisory references to GHSA-5hgr-hg42-57jg.

Audit refresh, July 8, 2026:

• Confirmed the live PR is open, ready for review, merge-clean, and the visible advisory check is green.
• Rechecked the merge-base diff: exactly one advisory JSON file changes (advisories/github-reviewed/2026/06/GHSA-5hgr-hg42-57jg/GHSA-5hgr-hg42-57jg.json), JSON parses, git diff --check passes, and the exact-head worktree is clean.
• Revalidated the added public references in the diff; all resolve through their source site or registry.

[cookesan]

Rechecked this reference PR while keeping the Advisory Database queue current.

• The current branch adds the NVD advisory reference for CVE-2026-49460 and the pypdf PNG predictor fix commit reference; it does not change affected ranges, package metadata, severity, aliases, or review metadata.
• NVD returns one matching CVE-2026-49460 record.
• The commit resolves to 6755d925fae26a79c2f76f5c911bcd728f75a952 with message SEC: Optimize _decode_png_prediction regarding memory and speed (#3806).
• The advisory JSON parses, git diff --check HEAD^..HEAD is clean, and visible PR checks are passing.