Fix lettre commit reference

[cookesan]

Fixes lettre commit reference in GHSA-qc36-q22q-cjw3.

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/2021/07/GHSA-qc36-q22q-cjw3/GHSA-qc36-q22q-cjw3.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]

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

• The patch only fixes the lettre commit reference; it does not change affected ranges, package metadata, or severity.
• The added commit resolves to 93458d01fed0ec81c0e7b4e98e6f35961356fae2.
• The commit subject matches the SMTP transport fix context: fix(transport-smtp): Fix transparency codec.

No other advisory fields are changed.