fix(lang): correct two wrong citation lines, add check-citations

Two of la.ini's LT.txt:<n> citations pointed at the wrong line -- the
Latin itself was right, only the pinned line was wrong:

- advent cited LT.txt:8631 ("Tempus Nativitatis"); the real "Tempus
  Adventus" heading is at 8609.
- ef-christ-the-king and [rank]'s own citation both pointed near
  "Dominica ultima Octobris" (12459) when the text they actually quote,
  "D.NI NOSTRI JESU CHRISTI REGIS" and "I classis", sits two and three
  lines further down, at 12461 and 12462.

ef-christmas-sunday-0 was marked PATTERN but LT.txt:8644 is the identical
string verbatim -- relabelled as a direct citation, not constructed.

Added tools/check_citations.py and `make check-citations`: for every
LT.txt:<n> citation outside a PATTERN block, confirms a +-2-line window
around line n actually contains the Latin text the citation claims,
rather than trusting each of the 38 citations by hand. Follows
check-schema/check-templates' own precedent -- docs/ is gitignored, so
the target prints SKIPPED loudly and exits 0 when docs/research/LT.txt
is absent, never a silent pass.

The checker's own teeth are proven three ways: replayed against the
pre-fix file it independently re-derives both corrections above; a fresh
mutation (redirecting one citation to an unrelated line) is caught and
reverted; the fixed file passes clean, 147 citations checked, 0 wrong.

0 files changed, 0 insertions, 0 deletions