docs(governance): register accessibility compliance

[bokelley]

Summary

Implements the narrowed 3.2 slice from #4740:

• registers accessibility_compliance in the canonical governance validation-category table
• shows it in sync_plans category readback and parameter documentation
• explicitly distinguishes technical accessibility governance from Content Standards content-suitability evaluation
• leaves structured per-asset creative failures to Creative errors: structured accessibility violations #6169

No Content Standards schema, calibration task, format schema, or governance lifecycle change is included; those mechanisms already exist.

Verification

• focused category placement test passes
• schema-link lint passes
• docs-only change correctly carries no protocol changeset

Closes #4740.

[aao-secretariat]

Ladon verdict: Approve

Approve — small, well-scoped docs PR registering accessibility_compliance as a canonical governance validation category.

What I checked:

• Schema↔docs coherence: category_id in sync-plans-response.json is an open string (no enum), and the referenced format.accessibility.wcag_level / requires_accessible_assets fields exist in format.json — no wire-shape edit required, no drift.
• Changeset correctness: a minor changeset is the right type for a non-breaking, additive governance-category registration. Not a breaking-class change, so the hard breaking-change escalation gate does not apply.
• No high-risk paths, no gated paths.

Decision path: No critical/high findings (row 1 n/a). gated_paths is false, so row 2 does not fire despite review_decision: REVIEW_REQUIRED. No deletions, no medium data-loss/schema/infra findings, no author-team gate, fewer than three mediums. The single finding is low (PR body verification note contradicts the added minor changeset — cosmetic, the changeset itself is correct). Falls through to row 9 → approve.

Nit (non-blocking): .changeset/register-accessibility-governance-category.md — the PR body claims it 'carries no protocol changeset' while a minor changeset is present; consider reconciling the PR description.

[aao-secretariat]

Ladon verdict: Approve

Approve — docs-only Normative (non-breaking) addition registering the accessibility_compliance governance validation category.

What I checked:

• No wire-shape change: category_id in sync-plans-response.json is a free-form string (not an enum), so adding a new category value is not schema drift.
• Field references (format.accessibility.wcag_level, requires_accessible_assets at core/format.json:591-604) are accurate against the schema.
• Docs-only, no-changeset is correct — docs/governance/** is outside PROTOCOL_SCOPED_PATHS.
• Canonical-set framing does not contradict the opaque categories_evaluated labels in check_governance.mdx.
• Content-presence test ships alongside the docs change.

Gates:

• gated_paths: false — row 2 does not apply despite review_decision: REVIEW_REQUIRED.
• No no-auto-approve team match (row 7 N/A).
• high_risk: false; no deleted/modified high-risk files.
• No critical/high/medium findings.

This is an Editorial/Normative non-breaking docs addition, not a Breaking-class change, so the human-ratification hard gate does not apply. No findings fired rows 1–8; falls through to row 9 → approve.