feat(devx): hold the census page's row N references to the table by SEAM, not by number

`check-system-context-census` held every `file:line` anchor on the page to the
tree, but nothing held the two other hand-maintained claims on the same page: the
page's own prose row references and the `why:` strings in `NON_READ_ANCHORS` that
name a row by number. A row inserted into the behaviour table renumbers every row
below it and silently falsifies every reference past the insertion point, with the
gate green — measured on #15687 (three references went stale) and #15395 (all
three became true again by coincidence, observed by nothing).

Each reference now declares the SEAM it is about — a literal occurring in exactly
one numbered row — and the gate resolves the seam, reads the number the reference
carries, and refuses when they disagree, naming the reference, its number and the
row the key resolves to. A ledger keyed by number would rot in the same edit.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012zGPuVVX3deAx9LdjK8jCk

Author: claude