feat(ui): communication and AI chat

[ubugeeei]

Tracks this family from #3134. Experimental @vizejs/ui workstream.

Outcome

Ship message, thread, and AI chat families: streaming tokens, tools, citations, abort, and accessible composition.

Authoring contract

• Public components are real .vue files. Handwritten render functions and JSX are not the public authoring model.
• Generated compiler output stays an internal artifact.
• Hover and completion must be enough to use the API without leaving the editor.
• Keep runtime script small. Prefer native HTML, CSS, and platform APIs.
• Extension is a first-class contract: parts, slots, CSS properties, and typed hooks.

Inventory

• Chat Message, Bubble, Attachment, Marker, Message Scroller
• Composer with IME, markdown, mentions, and file paste
• Streaming tokens, abort, retry, edit, and stale-stream discard
• Tool-call cards, citations, and structured result slots
• Thread navigation, reactions, and read markers
• Comment thread, mentions, emoji picker, activity feed, presence, notification preferences
• Live region policy for incoming messages without stealing focus

Dependencies

Depends on #4888 and #4910. Toasts are #4905.

Quality gates

This family is not done until it satisfies the parent definition of done in #3134:

• Canonical Vue SFCs with explicit <template>, <script setup lang="ts">, and scoped <style>
• kebab-case filenames for every public source, test, story, and fixture
• Every public prop, emit, slot, expose, CSS custom property, part, and data attribute documented; every option states @default
• Literal types for props, emits, slots, expose, and useTemplateRef
• Equivalent public contracts across DOM, SSR, and Vapor builds
• WCAG 2.2 AA, WAI-ARIA APG, keyboard, pointer, touch, focus, and dismissal where applicable
• LTR/RTL, locale, reduced motion, forced colors, and zoom
• SSR, hydration, deterministic IDs, Teleport, Suspense, islands, and no request-global state
• Colocated Vue Test Utils tests, Vitest browser-mode coverage, and Playwright visual regression
• Musea stories covering states, slots, parts, and presets
• Tree-shakable ESM subpath, exact sideEffects, and enforced bundle budgets
• Headless by default; visual style arrives only through opt-in presets

Testbed notes

Streaming SSR/island fixtures, cancellation tests, and generic slot types for tool results. Composer is an IME testbed.

Out of scope

Toast notifications belong in overlays. Audio/video attachments use the media family.