feat: add React Aria variant (aria-vega)

Summary

Add React Aria as a component-library variant of AI Elements, parallel to the
Radix (radix-vega) and Base UI (base-vega) flavors — so AI Elements works for
projects on the shadcn React Aria distribution, not just Radix/Base UI.

Same request as #383 (Base UI), same shape as #450 (which adds the Base UI variant,
stacked on the radix-vega migration in #445). This asks for the third first-party
shadcn base to get equal treatment.

Motivation

shadcn now officially ships a React Aria base:
https://ui.shadcn.com/docs/changelog/2026-07-react-aria

Projects initialized with shadcn init --base aria install react-aria-components
based primitives. On such a project, bunx ai-elements@latest fails outright —
the CLI resolves AI Elements' shadcn deps against the active base and errors on the
first component that has no aria variant:

   The item at https://ui.shadcn.com/r/styles/aria-vega/hover-card.json
   was not found. It may not exist at the registry.

So today AI Elements is unusable on an aria-based project without hand-porting every
element. Base UI is getting fixed in #450; React Aria has the same need.

Proposed solution (mirroring #450)

• packages/aria-ui — the shadcn aria-vega component set (react-aria-components),
  parallel to packages/shadcn-ui (radix-vega) and packages/base-ui (base-vega).
• packages/elements-aria + packages/examples-aria — elements/examples ported
  onto React Aria, at parity with the other variants.
• Docs library switcher — extend PreviewLibrarySwitch / useUILibrary to include a
  React Aria option.
• bump-ui — sync the aria registry too:
  shadcn add --all --overwrite -c packages/aria-ui.

Porting notes / gotchas (from a real radix→aria migration of an AI Elements app)

These are the concrete divergences a React Aria port must handle — sharing so the
variant is correct, not just compiling:

1. No asChild. React Aria components don't support asChild. Every
   <TooltipTrigger asChild>, <CollapsibleTrigger asChild>, <DialogTrigger asChild>,
   <Button asChild><Link/></Button> must be recomposed (e.g. buttonVariants() on the
   element directly, or render/Focusable patterns). This touches most elements
   (actions, artifact, checkpoint, branch, audio-player, plan, web-preview, commit, etc.).

2. Different composition & prop names. Content components are renamed and triggers
   wrap their content:

   • Tooltip: TooltipContent → Tooltip, no TooltipProvider; <TooltipTrigger>{btn}<Tooltip/></TooltipTrigger>.
   • Dialog: open→isOpen, DialogContent wrapper removed.
   • Popover/DropdownMenu: PopoverContent/DropdownMenuContent folded into
     Popover/DropdownMenu; trigger wraps trigger+content; item onClick→onAction,
     disabled→isDisabled.
   • Collapsible: open/onOpenChange → isExpanded/onExpandedChange.
   • Tabs/Select: value/onValueChange → selectedKey/onSelectionChange, item value→id.
   • Button: onClick→onPress, disabled→isDisabled.

3. useControllableState. Several elements import
   @radix-ui/react-use-controllable-state (chain-of-thought, reasoning, stack-trace,
   transcription, mic-selector, voice-selector). The aria variant needs a non-Radix
   equivalent (trivial local hook).

4. Missing primitives. The shadcn aria-vega registry currently has no
   hover-card and no menubar (both 404). AI Elements uses hover-card
   (attachments, context, inline-citation). The aria variant needs either upstreamed
   aria versions of these or documented replacements (e.g. hover-card→popover-on-hover,
   menubar→dropdown-menu).

5. buttonVariants + RSC. The aria button is a client component; if
   buttonVariants is used in a Server Component it must be exported from a non-client
   module.

Acceptance criteria

• bunx ai-elements@latest add <component> succeeds on a project with --base aria.
• All elements render at parity with the radix/base variants (a11y preserved).
• Docs switcher offers React Aria; bump-ui keeps the aria registry in sync.
• The gotchas above are handled or documented.

Related

• #383 — Base UI support request
• #450 — feat: add Base UI variant (the analogous implementation)
• #445 — radix-vega migration
• shadcn React Aria changelog: https://ui.shadcn.com/docs/changelog/2026-07-react-aria

Compare the analogous implementation in #450 and the existing packages/shadcn-ui and packages/base-ui variants first. Map the React Aria work across packages/aria-ui, packages/elements-aria, packages/examples-aria, PreviewLibrarySwitch, useUILibrary, and bump-ui, then review the listed composition and missing-primitive gotchas. Done means the aria base supports all elements, the CLI installation succeeds, the docs switcher includes React Aria, and the registry stays synchronized.