feat: a11y completion + AbortSignal + grouped options (v3.2.0)

[michaeljfalk]

Summary

Tier 1 of the agreed roadmap — three additive features, no breaking changes.

1. Accessibility (ARIA combobox/listbox pattern)

• Options get stable ids; the active row carries aria-selected="true" and is referenced by the input's aria-activedescendant (so arrow-key nav is announced).
• Input gains aria-controls (→ menu) and aria-haspopup="listbox".
• A visually-hidden aria-live="polite" region announces result counts, "Searching…", and the no-matches/create state.

2. AbortSignal for async sources

• Async source ctx now includes a real signal → { scope, limit, query, signal }.
• A newer search aborts the previous one's signal; remoteSource passes it to fetch; destroy() aborts in-flight too.
• Fixes a doc/code mismatch: signal was documented on line 27 but never actually provided.

3. Grouped options

• groupBy(option) => string function or per-option group field → <optgroup>-style headings (.liveselect__group).
• Results are stably reordered so same-group items sit together (first-seen group order), so sources needn't pre-sort. activeIndex/keyboard nav stay consistent with the reordered list.

Changes

• dist/liveselect.js — a11y wiring, AbortController per search, _group/_groupOf/_announce helpers, group field in normalizeOption, remoteSource passes signal
• dist/liveselect.css — .liveselect__group heading style
• test/client.test.js — 4 new tests (a11y ids/activedescendant/live-region, abort-on-supersede, grouping reorder, groupBy precedence) → 30/30 pass
• README.md — Grouped options + Accessibility sections, options-table rows, async-ctx note, Features bullet
• examples/vanilla.html — grouped-options demo (section 5)
• CHANGELOG.md + package.json — bump to 3.2.0

Test plan

• npm test → 30/30 (was 26); existing XSS/escaping regressions still pass
• Core + example inline script both parse clean

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