Pin accessibility gate across Knowledge-as-Code surfaces

[snapsynapse]

The canonical CI used a floating accessibility action and an errors-only threshold across the generated template, root landing, and canonical demo. This pins all three scans to A11y Audit v3.1.0 at immutable commit 6af2c95, selects the scanner's required Node.js 22 runtime, and applies the fail-closed major policy while preserving the generator's Node.js 18 and 20 compatibility matrix.

The accompanying source and generated-output repairs make contrast, link distinction, decorative structure, matrix labels, and theme controls deterministic for automated evaluation. The checked-in adoption receipt records the action identity, runtime boundary, route coverage, theme checks, and result digests.

Validation

• A11y scan: 45/45 required routes passed across template docs (22), canonical root (1), and canonical demo (22), with zero confirmed major findings, advisory findings, unknown-impact violations, blocking or advisory incomplete candidates, or scan errors.
• Theme coverage: 14 light/dark page-and-theme cases passed with zero violations and zero incomplete candidates.
• Repository gates: validation, verification, evals, initializer golden path, generated-output currency, link checking, offline search contract, and manifest hashes passed on the reviewed content.
• Runtime coverage: generator remains tested on Node.js 18 and 20; the scanner job uses Node.js 22.

Related to #2. Keep the issue open until hosted acceptance is verified after merge.