feat(spec): check what an objectui read-point anchor's cited line SAYS, not only its sha label (#16800)

`check:objectui-pin-citations` verified the sha spelling beside each read-point
citation. A record is a claim about a LINE'S CONTENT, though, so an anchor that
was wrong the day it was written — in a file byte-identical across a pin hop —
survived every sha refresh and every line-number refresh. Measured twice: two
such anchors in one re-read, three in the next.

Adds a content assertion an author may write beside an anchor — the quoted
first non-blank line of the cited range — which the gate checks against
objectui at the pin. On a mismatch it searches the file and NAMES the line the
quote really sits on, so an ordinary pin bump is a line-number edit and the
quote never moves; a diff that rewrites a quote is therefore a reviewable claim
that the read point changed.

The ordinary run does not require ../objectui (absent in CI and in sibling
worktrees) and never skips silently: it prints how many assertions it could not
verify. `--verify-anchors`, the pin-bump mode, goes RED when it cannot read the
pin, and prints the remaining coverage worklist — generated, never a roster.

Coverage is a ratchet, not a migration: ASSERTED_ANCHOR_FLOOR is an equality in
both directions, and no `gen:` fills a quote, because a quote copied without a
re-read reproduces this defect at scale.

Claude-Session: https://claude.ai/code/session_016N6xmWt5hYm94ffVEwGH8x

Co-authored-by: Claude <noreply@anthropic.com>

Author: zhuangjianguo