fix(a11y): unique modal title ids and close-time listener cleanup

[BrianGenisio]

Summary

Closes #19, #21, and #22 (D8, D10, D11 from the ChatCPT WCAG 2.2 AA audit).

Stacked or retained modal overlays no longer share id="modal-title", leave document keydown handlers attached after close, or ignore prefers-reduced-motion on in-content hash links.

Changes

Title ids follow the dropdown pattern: modal-title-${n} per instance, with aria-labelledby pointing at that id (omitted until a title exists).

Escape and focus-trap listeners attach in open() and detach in close(). Escape closes only the top of openModalStack, so a confirm dialog on top of settings does not dismiss both.

In-modal hash links use scrollIntoView({ behavior: 'auto' }) when prefers-reduced-motion: reduce. Default motion is unchanged.

Overlays still stay in the DOM after close, which is the existing reopen API. Unique ids make that safe. D9 (translated accessible names) is a separate contract change and is not in this PR.

Test plan

• Two constructed modals have distinct title ids; aria-labelledby matches
• Open A then B; Escape closes B only, then A
• close() clears document listeners; open() reattaches
• Help-modal TOC link uses smooth scroll by default and auto under reduced motion
• axe on an open titled modal in light and dark (npm test)
• Keyboard walkthrough on components/modal/test.html

ChatCPT does not hardcode modal-title. After merge, the app needs a submodule bump; no constructor API change here.

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📥 Commits

Reviewing files that changed from the base of the PR and between d21d15d and a17e029.

📒 Files selected for processing (3)

• components/modal/README.md
• components/modal/modal.js
• tests/modal-a11y.spec.js

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