fix(cli): audit text beneath transparent composition boxes

[user-github-me]

Visible text marked data-layout-allow-occlusion is skipped by the contrast audit when a later transparent composition box wins elementFromPoint. On current main, both a direct host and a mounted sub-composition reproduce check exiting successfully with zero contrast checks for text whose measured ratio is 1.57:1.

Walk the ordered hit stack through unpainted composition hosts and compiler-produced inner roots, then audit the text when a probe reaches it. Keep the existing behavior for covering paint, generated content, raster/replaced elements, and browsers without elementsFromPoint. Paint checks are cached only for the current prepare call.

Refs #5112 (contrast-audit follow-up). This addresses the audit problem; #5118 changes one registry block, and the font-readiness report remains separate.

Validation:

• Four regressions fail on main; the new 31-case suite and existing layout/contrast suites pass 221 tests.
• Real Chrome prepare/screenshot/finish checks: transparent and nested hosts report the 1.57:1 failure; opaque backgrounds, gradients, shadows, generated content and a deeper opaque cover remain excluded.
• Built CLI check: the direct and mounted cases now report three contrast failures across three timestamps. Readable-text controls pass all three checks; a fully opaque cover still passes with no covered-text contrast checks.
• CLI typecheck/build, repository lint, formatting, all applicable pre-commit gates, comment checks and test reachability passed.
• The exemption is deliberately limited to unpainted HTML div/section composition boxes. This is an audit correction, with no render changes. No claim that all transparent HTML/SVG paint or partial-cover contrast cases are solved.

[jrusso1020]

Thanks, this fixes a real gap. A transparent sub-composition host stacked above allow-occlusion text was answering elementFromPoint for the whole block, so contrast was never checked there. Walking the hit stack only through unpainted composition boxes, and stopping at the first painted layer, keeps every real cover intact. The tests cover host, nested-root and inner-root cases, plus each kind of paint that should still count as cover. Possible follow-up: isClippedAway uses the same single-hit probe, so clip-path text under a transparent host may still be skipped.

— Rames