fix(a11y): survive audit timeouts, and stop overclaiming CI floor coverage

Two problems the first CI run exposed.

Audit timeouts. Three tests failed with "Audit failed to complete in
time" (code -56) on the GitHub runner. That is the audit's own internal
deadline on a slower machine, not an app defect, and the harness had no
resilience to it. Audits now retry up to three times, and a screen that
still cannot be audited is reported via XCTSkip rather than passing.
Skips are distinct from passes in CI, so an unaudited screen stays
visible instead of being silently counted as clean. The Dynamic Type
sweep attempts every screen before skipping, so one slow screen cannot
drop the other four.

Overclaimed floor coverage. The two-simulator matrix was justified on
covering the oldest supported OS, but the macos-26 image ships only iOS
26.x runtimes, so "floor" resolved to 26.2 and "current" to 26.5 — the
run compared two 26.x images and never touched an 18.x one. The measured
non-nested coverage that motivated the matrix (18.6 vs 27.0) reproduces
locally but not on this runner. The workflow comment now states this
plainly, and the selection step emits a warning annotation when the
resolved floor sits a major version or more above the deployment target,
so the gap is visible in the CI UI rather than assumed away.

Installing an older runtime in CI is possible via xcodebuild
-downloadPlatform but costs several GB and minutes per job; left out
pending a call on whether that trade is worth it.

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