Count the particle chain's units in S2's read instead of merging a copy (#620) - #622

- run_start counts in the chain's units too: a particle opening a run
  counts once, its bound particle and joined words not at all ('John
  der la Jr. Prof. Smith' keeps #602's run; 'van la Smith Secretary
  Jr. Dr. Smith' starts none, as before #614).
- credential_run gathers adjacent particles on the read's path into
  the one piece the chain makes of them, so an absorbed run reports
  as before #614 ('Mc Mc'); M2's view still reports word by word.
- The numeral fork keeps reading the piece in front as written
  (rules.md#P2's 'John van der J. V'); 'John van B and Smith X' is
  back to 0c54662's reading.
- tail_reading's resume test needs no units clause: fuzzed over
  372,330 names, none differs; the reason is in the comment.
- Eight rows #614 labelled `parity` did not match 1.4.0; relabelled by
  bisect. Three new rows, two unit-mark tests, a report-token test;
  every new branch killed by a recorded mutation.
- Docs: rules.md#S2 (run start) and P2's implemented line,
  decisions.md#S2 (#620 entry) and parse-cost, mechanisms.md
  UNIT-PARTITION and READ-WITHOUT-THEN-BIND, AGENTS.md (parity means
  1.4.0). Pins: 307/344 on 3.11, 288/325 on 3.12-3.15.

Measured against 4278693: fingerprint 0, connective grid 20 (all back
to the pre-#614 reading), S2 grid 640 (508 back, 128 Freiherr-chain
acronyms by the two-name-words invariant, 4 one garbage name).

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

The review found the first count wrong where both masters were right:
marking every word the chain reaches JOINED let a member the chain
would join count as no word in front of the next ('Freiherr von Berg
MA X.Y.Z.' kept 'MA X.Y.Z.' in the family), and let a unit made only
of words the read then takes move the name's start into the titles
('St St VI', 'Freiherr St MA' read the numeral or acronym as the whole
family, unreported).

- _weighed: a word the trailing read weighs (suffix word, ambiguous
  class, by-shape member, initial, roman numeral by shape, period-
  marked title) keeps a unit of its own, as a position counted it;
  only plain words behind a particle run are JOINED, and a unit with
  nothing past its opener moves no name start.
- Against master as merged: fingerprint 0, connective grid 0, S2 grid
  12 (three garbage names). Frames equal master's; the 3.12-3.15 pin
  is corrected to 289/326, stale since #614's /simplify.
- Rows for the review's regressions and for its six test gaps (a title
  spliced off before the unit count, a P3-joined particle phrase), two
  report-token asserts, killers for each _weighed arm; every new
  branch dies under a recorded mutation.
- Docs rewritten for the redesign: decisions.md#S2's #620 entry and
  parse-cost, rules.md#S2, mechanisms.md (numeral fork reads as
  written; the copy's defects were patched, not live), the anchor
  comment, AGENTS.md (an exclusion list being replaced carries rules).

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

/simplify pass, behavior-preserving: the suite, the five gates and
the fingerprint, connective and S2 grids (byte-identical) unchanged;
frames unchanged (308 on 3.11, 289 on 3.12).

- tail_reading looks up where the second unit opens once and hands it
  to every peel pass. Each pass had rescanned from the front, a
  C-level quadratic behind a long joined surname (5.77x for 4x the
  input against master's 4.08x, now 4.03x) that no frame guard saw;
  test_the_second_unit_is_found_once_per_read counts the lookups.
- The peel stops at any piece the chain joins into a run, not only a
  bound particle: a joined word the reading does not weigh was going
  to break the walk anyway, after the suffix test's frames.
- credential_run gathers an absorbed particle run off the BOUND marks
  instead of re-testing particles and adjacency; run_start's two
  counting branches are one condition.
- _weighed is pinned to the reading: no piece marked JOINED is ever in
  the run the read takes (negative control: 320 grid names).

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