Grounded-or-silent reference assistant for public agencies: answers only from a
supplied corpus, cites everything, and refuses cleanly when it cannot ground an
answer.

The index now carries a fingerprint of the corpus it was built from, so an
index that cannot say what it came from is refused rather than trusted. That
closes the case where editing a document without re-indexing made cairn quote
stale text under a citation resolving elsewhere.

Verified at this tag: 356 tests, 63 browser checks, ruff clean, gate PASS,
guard PASS, and the committed bundle re-records byte for byte.