Clarify footer update dates

[shaug]

TL;DR

Replace cryptic footer hashes with linked ISO data/site dates while keeping the Sources page's panel version visibly auditable.

Summary

• Show separate Data last updated and Site last updated ISO dates linked to the exact build commit across HTML and PDF output.
• Derive the data date from the editorial cutoff and the site date from the checked-out commit timestamp without weakening deterministic builds.
• Keep the Sources page's panel ID and short hash visible, linking the hash to the public release-status record containing the full provenance.
• Preserve exact Git, panel, and artifact identities in the public release status and manifests.

Validation

• make check
• Repository review suite: solution simplicity, correctness, and code simplicity all clean on f4a80b18d6de05c72f3b7a30d24c1c2404c11a9c

Context: #130