Check ledger rule citations as verbatim excerpts, fix the stale ones - #632

The first commit checked only the colon form's first quote, and the
review of #632 found the rest of the class: `rules.md#X -- "..."`,
`rules.md#X's Accepted clause ("...")`, `rules.md#S2 consumes "..."`
and a second quote chained by "and" all passed unread, 23 of them
stale. The scanner now:

- reads a quote after the ID in any of those shapes (_LEAD_RE), and
  every quote chained to it; a reference with no quote must still
  name a real rule, section, mechanism or decisions entry
- quotes against a rule's statement AND its Accepted: clauses (example
  and pointer lines excluded), a section letter's Background
  (`rules.md#H Background`), a decisions entry's text
- matches lowercase decisions keys, merges the `### differential-
  ledger, ...` arc headings onto their one key, rejects empty "[...]"
  fragments, reads `--` as the em dash and a nested ' as ", follows
  `#:` comment continuations and IDs wrapped at a hyphen
- skips this module's own illustrative comments
- records the retired excerpts as a negative control, and asserts the
  sweep reaches each shape (non-colon, chained, decisions, elided)

The colon form alone still defines a citing module for implemented:.

Fixed: P5 (the reserve's numeral clause and the join-after-the-run
wording, #614), P2 (S2's once-read run), M2 (#601 replaced the walk
with the clause-free trailing run; the 'Jane Doe Jr. nee Smith Ma'
paragraph gains a dated note that #601 moved it to fix(#601/#602)), S2
("OPENS THE STRING"), A1's segmenter clause, a decisions.md#S2 citation
of rules.md#S2's Accepted clause, and four ledgers crediting AGENTS.md's
"a reader would hesitate too" to rules.md#A1. Four ledgers' paraphrase
of decisions.md#S2 now quotes its headline and names #544's second
exception. With Accepted text quotable, the first commit's H5 and S2
rewrites go back to the original, more exact Accepted-clause quotes.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

Scanner (tests/v2/test_doc_citations.py):
- a section's Background is read whole, not its first line; a wrapped
  no-boundary: marker's continuation lines are no longer quotable
- each citation records its lead, and the reach test asserts every
  lead shape is live ('s, parenthesis, dash, bare words) rather than
  any one of them; a new test pins what is quotable (an Accepted
  clause, a Background's wrapped line) and what is not (an example
  line, a no-boundary: continuation)
- the docstring states the limits: case folds, and a quote more than
  four words from its ID and chained to nothing is not checked -- the
  measured alternative, every quote in the paragraph, flags 139, nearly
  all names written in double quotes
- _RETIRED_EXCERPTS drops its P6 row (no version ever routed a
  decisions quote to a rule) and says which direction it guards

Ledgers: fourteen stale quotes the check does not reach, found by the
review -- C1's paired-initials sentences in all five (#563 made the
speaking word an UNAMBIGUOUS suffix), N3 in three, P2 in one. Dated
landing paragraphs for P2 (#424) and P5 (#425) now say they describe
the rule as it then stood instead of quoting today's text as their
reason; the M2 rule's opener names all four names #601 moved, not one.

AGENTS.md: the count is measured, not summed from commit messages --
27 stale quotes and 5 quoteless colon citations, the current check
over e4b653a -- and the sentence states the four-word reach.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

e4b653a lived on #631's branch, which the squash merge leaves to be
deleted; 8e6e524, master after #631, has the identical tree.

Co-Authored-By: Claude Opus 5.5 <[email protected]>