fix: keep the pin cite on an Id. citation that refers to a section

[aseem-ai]

Fixes

Refs #299 — deliberately not Fixes, see the note at the end.

Summary

Id. § 1985 lost its pin cite, and the section mark was reported as a
separate UnknownCitation. A plain Id. at 5 was always fine, so this only
hit Id. cites pointing at a section.

The cause is the one named in the issue: the mark is its own token, and
match_on_tokens() stops at any non-string token when strings_only=True,
so the scan never saw past it. This is fix option 2 from the issue.

Two changes in helpers.py:

• match_on_tokens() reads through a SectionToken instead of stopping at
  it. The mark is a marker, not a citation, and its text is already pin cite
  vocabulary — PIN_CITE_TOKEN_REGEX has listed §{1,2} as a label all
  along, it just never got to see one.
• filter_citations() drops an UnknownCitation that overlaps the citation
  before it. Once the mark is part of the pin cite it is not a citation of
  its own, and this also silences the "Unknown overlap case" warning the
  pairing was logging. A mark inside a parenthetical is still kept, by the
  rule directly above.

Tests

python -m unittest discover -s tests -p 'test_*.py' → 56 tests, OK.

New test test_id_citation_with_section_pin_cite fails on main (the stray
UnknownCitation is the extra element) and passes here. It covers
42 U.S.C. § 1983. Id. § 1985., Id. §§ 5-7., the glued Id. §5. form,
Id. § 5, 7., a guard that Id. at 5 still works, and a guard that a mark
no citation absorbs is still reported on its own.

match_on_tokens() is shared, so I read its six call sites and diffed
behaviour on the neighbours, including the backward scans for party names
and supra antecedents: § 3.1 (2d ed. 1977), Strawberry Hill, 725 S.W.2d at 176, 1 U.S. 1 (citing § 5), § 5. Adarand, supra, at 240, lorem ipsum see §99 of the U.S. code., Fla. Stat. § 120.68 (2007), Mass. Gen. Laws ch. 1, §§ 2-3. All byte-identical before and after. Extraction over
tests/assets/opinion.txt is unchanged at 170 citations.

Why this is Refs and not Fixes

The example in the issue, Id. § 394-ccc(2)-(3), still does not produce a
pin cite after this change — but for an unrelated reason. 394-ccc is a
section number with letters in it, which is #146, and #331 is open for it.
Widening PIN_CITE_TOKEN_REGEX to swallow that shape would put a statute
section grammar inside the pin cite every case citation uses (at 5-ccc(2)
would start matching) and would overlap #331, so I have left it alone.
Say the word if you would rather this close #299 and track the lettered
form solely on #146.

AI Disclosure

• No AI tools were used to create the content of this PR.
• Parts of this PR were created with the help of an AI tool, and I have carefully reviewed all of its content and take full responsibility for it.

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