Add DOMPurify selectedcontent fixed references

[cookesan]

Adds DOMPurify selectedcontent fixed references to GHSA-87xg-pxx2-7hvx.

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-87xg-pxx2-7hvx/GHSA-87xg-pxx2-7hvx.json), JSON parses, git diff --check passes, and the exact-head worktree is clean.
• Revalidated the added public references and package-version evidence in the diff; all resolve through their source site or registry.

[cookesan]

Revalidated July 8, 2026: this remains scoped to DOMPurify 3.4.5 fixed evidence for GHSA-87xg-pxx2-7hvx. npm confirms dompurify@3.4.5, the upstream release and source tag resolve, and GitHub's advisory record maps the GHSA to CVE-2026-47423. Local merge-base checks pass with a single advisory JSON file changed, JSON parsing, git diff --check, and a clean worktree.