feat(a11y): engage the audit enforcement ratchet (#21)

The point of the Phase 0 harness finally lands: named findings in
textClipped, dynamicType, hitRegion, elementDetection,
sufficientElementDescription, and trait now FAIL the build on the
empty-state suite. Regressions in five phases of accessibility work are
gated, not narrated.

Three carve-outs, each earned by evidence rather than convenience:

- contrast stays report-only. The two long-standing Settings findings are
  rows scrolled under the translucent tab bar, and their attribution
  flips between a row name and nil run-to-run — no suppression is narrow
  enough to keep CI stable. The centralised palette is the real guard.
- The seeded dense-row tests run reportOnly. Bisection showed the audit
  degrades on children-ignored content — the correct VoiceOver treatment
  for dense rows — emitting unattributed contrast/dynamicType failures on
  rows that measure 6-7:1 and render correctly.
- Characterised noise is suppressed narrowly and always logged as
  [noise: reason]: disabled controls (WCAG 1.4.3 exempt), "nearly passed"
  near-misses, system field placeholders (flagged at any length — proven
  by shortening them to no effect), and unattributed clipped/dynamic-type
  artifacts. noiseReason(for:) records each rule's provenance inline.

Validated in both directions. Positive: the full 11-test suite passes
with enforcement live, dark and light, on an erased simulator. Negative:
re-injecting the Phase 3 icon-exposure regression produced two named
[FAIL] findings and a failed suite — on both screens sharing the
component — then went green again on revert.

Docs and the CI workflow comment updated to describe the engaged state.

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