tooling(check-adr-anchors): verify a cited Dk names a real decision of the cited ADR (#12785)

Extends `scripts/check-adr-anchors.mjs` one layer below the number: for every
`ADR-NNNN Dk` citation on the anchored surface (each anchor's `invariant` and
each anchored file's text), `Dk` must appear in the cited record's decision
index. The number layer already proved 0057 names one record; nothing read the
`D10` half, which is the #8386 / #9255 failure family.

The heading grammar was measured across `docs/adr/**` before the parser was
written, and the check recognises exactly the four shapes that exist: `### Dk`
headings, `**Dk —**` bold leads, `| Dk |` decision-table rows, and a `Dk`
heading's own top-level list declaring `Dk.1`/`Dka` sub-decisions. That last
rule is load-bearing: 20 live citations into ADR-0120 D5's lettered gates and
ADR-0020 D3's numbered steps resolve only through it.

"Cannot verify" is a third verdict and is printed, never folded into the OK
line: 67 of 125 ADR numbers declare no machine-readable decision letters, and
an unrecognised shape must not read the same as a clean one.


Claude-Session: https://claude.ai/code/session_01PfaSTikked61BkcsB5Rn69

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

Author: os-zhuang