feat(Kanban): add Kanban compound component over createKanban

[johnleider]

closes #518

Adds the Kanban compound component over createKanban (part of #509).

Anatomy

Part	Role
Kanban.Root	Creates the createKanban board and a pointer-only useDragDrop context; owns the keyboard move model; emits move
Kanban.Column	Registers a column; v-model is the column's item array; disabled and accept(value, from, index) gate moves
Kanban.List	Drop zone (role="list") — direct parent of the column's items, exposes data-over
Kanban.Item	Movable item identified by value (role="listitem", data-state="grabbed" | "dragging" | "idle")
Kanban.Instructions	Keyboard instructions every item references via aria-describedby
Kanban.LiveRegion	Polite live region for pick-up / move / drop / cancel / rejection announcements

The anatomy follows the composable's two tiers (board → column → item). List is split from Column because useDragDrop resolves the drop index against the zone element's direct children, so column headers and counters must live outside the zone.

Design notes

• Data flow. The bound array per column is the source of truth. External changes (push, splice, replace) reconcile into the column's sortable; board moves write the new order back. Item identity is the value itself (===), so values must be unique across the board. Column reconciles through register / unregister / reorder, falling back to offboard + onboard when reorder is gated by disabled.
• Keyboard, not drag-only. The default KeyboardAdapter nudges a pixel point, which can't express "one slot" or "next column" reliably, so the component installs only PointerAdapter (4px threshold so clicks on inner controls don't start drags) and implements a discrete model. Space/Enter picks up; arrows and Home/End move a virtual drop target without touching the board (RTL-aware; axes swap for orientation="vertical"). Cross-column steps skip columns that are disabled or whose accept(value, from, index) refuses the item, so keyboard users reach every column a pointer drag can. The target is previewed through data-target on the column and list and data-drop="before" | "after" on the neighbouring item, and each step is announced. Space/Enter commits a single transfer; Escape and blur discard the target with no board mutation. The pick-up is released when its item leaves the board and re-anchored after external changes. Focus follows the item when a drop remounts it in another column.
• Events. move fires once per drop with { value, from, to, fromIndex, toIndex }, for pointer and keyboard alike, same-column and cross-column.
• Locale. New Kanban.* strings in locale/messages/en, including aria-roledescription.
• No composable changes. createKanban covered everything needed (transfer, per-column disabled / accept, browse for value lookup).
• Size limit. useTheme already measures 8.56 kB on dev (the size job failed on feat(Otp): add Otp compound component over createOtp #787), so this carries the same 8.5 → 8.6 kB bump as feat(Virtualizer): add Virtualizer compound component over createVirtual #788 to keep the job green.

Checklist

• packages/0/src/components/Kanban/ + barrel + components/index.ts
• maturity.json: draft → preview, since: null, description
• Docs page components/data/kanban.md + typed-router entry, examples (basic, wip-limit, move-log), components index entry, README
• surface.test.ts, fixture + a11y.browser.test.ts entry (0 axe violations)
• Unit (index.test.ts, 40 tests) and browser (index.browser.test.ts, 10 tests — real pointer drags, focus retention, keyboard skipping a rejecting column, RTL)
• Changeset (@vuetify/v0: minor)