feat: add MultiSelect composition

[isaquebock]

Summary

• New inputs/multi-select composition aligned with Figma 3899:29362 (options panel) and reusing the InputText / Select trigger visuals.
• Root MultiSelect plus four sub-components: MultiSelectTrigger, MultiSelectContent, MultiSelectGroup, MultiSelectOption.
• Multi-only API — no multiple prop, modelValue: unknown[] defaulting to [], displayValue: (value: unknown[]) => string. The option indicator is the existing webkit Checkbox; semantics ride on role="option" + aria-selected. Content is role="listbox" with aria-multiselectable="true". Enter / Space toggle without closing the panel; Escape and Tab close it.
• Popover rendered with native primitives only — <Teleport> to body plus getBoundingClientRect + fixed positioning + the popup-scale-in/out semantic animations. No @floating-ui runtime.
• #search and #footer slots on <MultiSelectContent> are optional and ungated by props (presence drives rendering).
• Styling lives inline on each root class with data-* variants, per .claude/rules/styling.md.
• Storybook: Default, Sizes, WithGroups, WithSearchAndFooter, WithOptionExtras, LongList, Filled, Invalid, Disabled — every story binds args so controls drive the component.
• Five new webkit exports: ./multi-select, ./multi-select-trigger, ./multi-select-content, ./multi-select-group, ./multi-select-option.

Pending follow-ups (tracked, not in this PR)

• var(--bg-surface-raised) is used on MultiSelectContent matching the existing SelectContent precedent but is not yet catalogued in .claude/docs/DESIGN.md. Decide whether to catalog the token or migrate both panels to var(--bg-surface) + var(--shadow-xs).
• No multi-select.figma.ts yet — the select family has no Code Connect precedent. Adopt Code Connect for Select first, then mirror.

Test plan

• pnpm webkit:lint && pnpm webkit:type-check && pnpm webkit:type-coverage
• pnpm --filter storybook build
• Storybook → Components / Inputs / MultiSelect: each story renders; controls panel drives the component.
• Keyboard: open with Space / Enter / ArrowDown, navigate with Arrow* / Home / End, toggle with Enter / Space without closing, close with Escape.
• Open the panel and inspect: <div role="listbox" aria-multiselectable="true">; selected option has aria-selected="true" and an internal <Checkbox aria-hidden="true">.
• Resize / scroll the page with the panel open — anchored position updates.