feat(a11y): add accessibility audit harness (#21 phase 0)

Phase 0 of the accessibility pass: a regression guard that must exist
before any of the remedial phases, so their acceptance criteria are
enforced rather than asserted once by hand.

- Fix the project-level IPHONEOS_DEPLOYMENT_TARGET, which was 26.2 while
  all three targets are 17.6. It was shadowed everywhere today, but any
  target added later would silently inherit it and drop iOS 17.6 support
  with no error.

- Add a DomainDigUITests target running performAccessibilityAudit on the
  six primary screens, plus a sweep of every root screen at
  AccessibilityXXXL. Uses the existing DOMAIN_DIG_FORCE_PRO_PLUS debug
  argument so Pro-gated screens are reachable.

- Findings are reported, not failed. The audit surfaces violations that
  exist today, so gating on them would block unrelated PRs until the
  whole pass lands. Enforcement is a committed constant,
  AccessibilityAuditHarness.enforcedAuditTypes, widened per audit type as
  each phase clears a category.

- CI now runs xcodebuild test across two simulators. Audit coverage is
  not nested between OS versions: on Tracked Domains, iOS 18.6 reported 2
  findings and iOS 27.0 reported 6 (including contrast and
  element-detection issues 18.6 never raised), while at accessibility
  text sizes the Dashboard produced a hit-region finding on 18.6 that
  27.0 did not.

- Simulator selection is now dynamic and floor-aware. The previous
  selector took the first iPhone from any runtime, which can resolve to a
  simulator below the deployment target where the app cannot install.

Baseline on iOS 18.6: 15 findings across 7 tests — text clipping on every
screen, contrast on Inspect and Settings, and a hit-region failure on the
Dashboard at accessibility text sizes.

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