Renumbering detection

[houfu]

From ROADMAP.md § M2. PRD: R11, D7 (ADR-0009)

Matched content with a differing label is reported as a renumber, not as inline text edits of the label. It falls out of label-vs-content matching in #131: the label pass and the content passes disagree about a pair, and the disagreement is the signal.

The sample pair's inserted clause (PRD § 3a) renumbers every clause below it and a cross-reference is updated to follow. The expected result is one insert, a run of renumber nodes with old and new labels, and a modify on the cross-reference block whose span carries the new referenced label. Checked by the golden in #144. Renumber recall is one of the four benchmark metrics (#143).

Depends on #131.