feat!: multiple selection (v4.0.0)

[michaeljfalk]

Summary

The headline feature: multiple selection via multiple: true. The major version marks the size of the addition — single-select behavior is unchanged and there are no breaking changes (everything new is gated behind the flag).

Multiple selection

• Chips for each selection. Add by clicking a row or typing + Enter; remove via a chip's × or Backspace on an empty input; re-selecting a chosen row toggles it off.
• Array value throughout: getValue() → string[], getOption() → option[], onChange(values, options), setValue([...]), and liveselect:change detail { name, value: [], options: [] }.
• maxItems caps selections and suppresses the create row at the cap.
• submitFormat controls plain-form submission: 'repeat' (default — one hidden input per value sharing the name, exactly like native <select multiple>, so Express parses req.body.tags as an array), 'bracket' (name[], PHP/Rails), or 'delimited' (one input joined by delimiter).
• A11y: chosen rows get aria-selected + .liveselect__opt--chosen; the listbox is aria-multiselectable.
• enhance() auto-detects <select multiple>, upgrades to multi mode, and keeps the original element's selected options in sync.

Changes

• dist/liveselect.js — multi state; chips container + per-submitFormat hidden inputs; _toggle/_deselect/_renderTags/_afterMultiChange/_syncHiddenMulti; multi branches in _select/_emit/_syncInput/_syncHidden/_syncValidity/_canCreate/getValue/getOption/setValue/clear; Backspace-to-remove; chosen-row marking; enhance() multi support
• dist/liveselect.css — chips/control layout, chosen-row checkmark
• test/client.test.js — 9 new tests → 45/45 pass
• README.md — Multiple selection section, options table, Events/Instance API notes, Features bullets
• examples/vanilla.html — multi-select demo (section 6)
• CHANGELOG.md + package.json — bump to 4.0.0

Test plan

• npm test → 45/45 (was 36); all single-select regressions (XSS, a11y, validation, groups, etc.) still pass
• Core + example inline script parse clean

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