fix(tools): close five more ways to defeat check_citations.py

Round 1 hardened check_citations.py against its own self-poisoning bug;
a review defeated it again. Five fixes, in the order they were found:

1. PATTERN silenced a whole comment block, not just the entry it was
   attached to -- a wrong citation on a DIFFERENT, unmarked entry in the
   same block (e.g. [season]'s own back-to-back trailing-comment style)
   was never checked at all. Fixed by scoping PATTERN with the identical
   leading/trailing pooling rule citations already use: an entry is
   excluded only by its own marker, never a neighbour's.

2. Explicit per-citation ranges (introduced in round 1 to replace a
   blanket +-2-line tolerance) had no upper bound, reintroducing the same
   defect at a much larger radius (LT.txt:8600-8650 passed if the text
   appeared anywhere in fifty lines). Capped at MAX_RANGE_WIDTH (3 lines);
   anything wider is reported MALFORMED, naming the entry and the width,
   instead of silently accepted.

3. The "pool too thin to verify" gate counted words, not rarity -- it
   flagged 11 genuinely correct citations (short Latin hagionyms with only
   one non-stopword) CANNOT VERIFY, while a match on nothing but "classis"
   (507 occurrences) passed freely alongside three siblings. Replaced with
   a frequency table over the whole LT.txt corpus: a token's evidence is
   1/(times seen), an item's evidence is its single rarest matched token
   (not a sum -- summing would let several merely-common words add up to
   "enough" between them, the same shape as the self-poisoning bug).

4. "LT.txt:12,459" (a comma typo for one number) parsed as two unrelated
   bare citations, 12 and 459, either of which could coincidentally match
   while the intended line was never checked. Detected as a
   thousands-separator-typo shape (a 1-2 digit token immediately followed
   by an exactly-3-digit one -- the only way a real LT.txt line number,
   which never exceeds 5 digits, splits under one comma) and rejected as
   malformed.

5. The self-test suite overstated its own coverage: of round 1's seven
   fixture cases, only two actually failed against the pre-round-1 script.
   Every test is now labelled REGRESSION or CHARACTERISATION, each
   verified by direct replay against the named prior version rather than
   asserted -- 14 of 33 are genuine regression tests.

Both of the review's own defeats (block-wide PATTERN silencing, the
50-line range) are reproduced as dedicated fixtures and confirmed caught;
both are also confirmed to slip through the pre-round-2 tool unchanged.

Claude-Session: https://claude.ai/code/session_017ZBxCCRM2ojnBupp3SBxV9

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