feat: Reorderable tabs (reorder, pinned, add-tab, cross-list)

[ernst-dev]

Description

Adds opt-in tab reordering to the Tabs component, reusing the existing @dnd-kit SortableArea / InternalDragHandle stack already shipped for List. Four capabilities, all additive and opt-in:

1. Within-list reorder — reorderable + onReorder({ tabIds }). Pointer drag and full keyboard support (Space/Enter to lift, arrow keys to move — RTL-aware, Esc to cancel), with live-region announcements.
2. Pinned / position-locked tabs — Tab.disableReorder keeps individual tabs fixed while the rest reorder around them.
3. Add-tab affordance — addTabButton + onAddTab render and handle a trailing add-tab ("+") control.
4. Provider-less cross-list drag — reorderGroup + onTabMove(TabMoveDetail). Two or more reorderable Tabs that share the same non-empty reorderGroup can exchange tabs with no wrapper or provider. Each Tabs keeps its own DndContext; coordination happens through a module-level AsyncStore registry. Supports Ctrl+Arrow keyboard transfer between lists and a geometry hit-test for pointer transfer via public @dnd-kit callbacks. onTabMove is a controlled event — the consumer updates both lists' tabs arrays (remove from source, insert into target); it fires on both instances involved.

List and provider-less (non-reorderGroup) Tabs behavior is unchanged — everything new is gated behind the new optional props.

New public API surface: TabsProps.reorderable, TabsProps.onReorder, TabsProps.Tab.disableReorder, TabsProps.addTabButton, TabsProps.onAddTab/AddTabDetail, TabsProps.reorderGroup, TabsProps.onTabMove/TabMoveDetail, plus new i18nStrings reorder/cross-list live-announcement keys.

Accessibility

• Keyboard model — within a list: Space/Enter lifts a tab, arrow keys move it (RTL-aware), Esc cancels and restores the original position. Across lists in the same reorderGroup: Ctrl+Arrow transfers the lifted tab into the adjacent list (also RTL-aware).
• Live-region announcements — grab/move/commit/cancel are announced within a list; a dedicated liveAnnouncementTabMovedAcrossLists string announces the moved tab's target position and total in the destination list.
• role="application" — the header switches to role="application" while reordering is active so the arrow-key interaction model is conveyed to assistive tech, and reverts afterward.
• Focus / selection handoff — the moved tab keeps focus and selection across the list boundary during a cross-list move so keyboard users don't lose their place.
• Reduced motion — drag/reorder transitions respect prefers-reduced-motion.

Related links, issue #, if available: API proposal (Chorus): https://chorus.aws.dev/doc/dB5QkUyuhbqU

How has this been tested?

Local verification on this branch (Node 24):

• Type-check: tsc clean.
• Lint: eslint clean; stylelint clean (both run via the pre-commit lint-staged hook).
• Unit tests: Tabs suite 107 / 107 passing.
• Full suite: 177 / 177 across 14 suites covering internal/components/sortable-area, list, and tabs — including the new cross-list-move, reorder-group-registry (registry lifecycle), and cross-list-reorder suites.
• Regression: existing List / SortableArea behavior intact (their suites still green).

Not yet done (honest status):

• WebDriver / integration (motion + real-DOM drag) tests — not yet written or run.
• Public API sign-off — PENDING. The new props/events above still need design + API review before this can leave draft.

Reviewers can exercise the cross-list behavior via the new dev page pages/tabs/cross-list.page.tsx (two reorderable Tabs sharing a reorderGroup), and run the unit tests with the Tabs / sortable-area / list Jest suites.

Note: per CONTRIBUTING.md, this repository currently accepts external code contributions only for bug fixes, and changes are expected to come from a fork against main. This is a feature opened as a draft for internal review and public API sign-off — not a ready-to-merge external contribution.

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates. — In-code JSDoc added for every new public prop/interface and new i18nStrings keys. Customer-facing website Usage-tab docs are still pending.
• Changes are backward-compatible — every new prop is optional and opt-in; List and provider-less Tabs are unchanged. See CONTRIBUTING.md.
• Changes do not include unsupported browser features — reuses the existing @dnd-kit SortableArea/InternalDragHandle stack already shipped for List; no new browser APIs. See CONTRIBUTING.md.
• Changes were manually tested for accessibility — keyboard model and live-region announcements are implemented and covered by unit tests; a full manual screen-reader / AT sweep is still pending. See accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function. — N/A: this change does not handle URLs.

Testing

• Changes are covered with new/existing unit tests? — Tabs 107/107; 177/177 across 14 suites, incl. new cross-list-move, reorder-group-registry, and cross-list-reorder suites.
• Changes are covered with new/existing integration tests? — WebDriver/integration tests not yet added or run.

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