Define and seed the reusable DSH accessibility testkit

[mattheliu]

Outcome

Turn repeated accessibility assertions into a development-only testkit with stable fixtures and versioned output.

Work

• Write an RFC for API boundaries, fixtures, report schema, supported runners, versioning, and false-positive handling.
• Seed reusable tests for names and roles, focus lifecycle, composite keyboard patterns, live regions, reflow, forced colors, reduced motion, and task flows.
• Keep native assistive-technology drivers outside the runtime package.
• Create a separate repository only after the first reusable code and ownership boundary are ready.

Acceptance evidence

• RFC and initial code are reviewable in this repository.
• At least one DSH core surface and the companion consume the same test helper.
• Output records exact DSH, browser, OS, and testkit versions.
• Documentation states that deterministic results are not screen-reader certification.

[mattheliu]

Initial reusable boundary seeded; RFC acceptance still open

PR #19 introduced the versioned reusable non-AT browser assertion helper. PR #20 adds a separate development-only hermetic AT lab launcher and structured result protocol:

• PR test: add versioned non-AT browser gates #19 browser helper/evidence: test: add versioned non-AT browser gates #19
• PR feat: publish DSH accessibility validation foundation #20 AT lab: feat: publish DSH accessibility validation foundation #20
• exact lab-readiness evidence: feat: publish DSH accessibility validation foundation #20 (comment)

The AT launcher stays outside the runtime entry points, records exact DSH/companion/OS revisions, uses synthetic fixtures, and explicitly refuses to convert deterministic startup into screen-reader certification.

This is useful seed code but does not close #11: the public testkit API/report-schema RFC is still required, one DSH core surface and the companion must consume the same extracted helper boundary, false-positive/versioning ownership must be reviewed, and only then should a separate repository be considered.