Add Cargo sparse registry advisory references

[cookesan]

Adds Cargo sparse registry advisory references to GHSA-p688-r7jv-fm6f.

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-p688-r7jv-fm6f/GHSA-p688-r7jv-fm6f.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-only PR while keeping the open Advisory Database queue current.

Adds direct upstream references for GHSA-p688-r7jv-fm6f. The advisory already references NVD, upstream PR #17031, the Rust blog post, package source, and the security announcement; this PR adds the relevant Cargo commit and the release-branch PR that contains that commit.

Current audit, July 8, 2026:

• GHSA-p688-r7jv-fm6f is live and not withdrawn.
• CVE-2026-5222 resolves in NVD with Analyzed status.
• Commit 3c51f26a26501ac51fd593db0d21f88719c65b1b resolves in rust-lang/cargo with subject CVE-2026-5222: avoid stripping .git suffix when for non git registries.
• The commit is associated with PR #17030 on the rust-1.96.0 branch; the advisory also keeps upstream PR #17031 for the mainline fix.
• crates.io cargo 0.97.0 resolves and is not yanked.
• Local JSON validation and whitespace checks pass, and the diff only changes this advisory.

No advisory data fields are changed.