Accessibility: add a blocking pa11y check for HTML surfaces

[ChelseaKR]

Goal

Add a blocking automated accessibility check for the browser playground and a
fixture-generated HTML report.

docs/CONFORMANCE-GAPS.md names the absence of axe-core or pa11y coverage as an
open accessibility gap. This issue addresses that specific gap.

Requirements

• Use deterministic synthetic fixtures.
• Exercise both web/index.html and a generated report.
• Fail CI on serious accessibility violations.
• Do not describe the automated result as a screen-reader attestation.
• Keep the local command reproducible through the Makefile.

Done when

• A local Make target runs the accessibility check.
• CI calls the same target.
• At least one regression fixture proves the check can fail.
• docs/CONFORMANCE-GAPS.md and the README conformance statement are
  updated accurately.
• make verify passes.