feat(chat): add timestamp, markdown, and streaming parts for agent use

[pasqualevitiello]

Stacks on #897. Ticket 04 of the Agent UI work: the parts agent conversations need, added to coss chat rather than forked into a new agent family.

Summary

 apps/ui/registry/default/chat/
+├── chat-message-timestamp.tsx   # <time> with machine dateTime, caller label
+├── chat-message-markdown.tsx    # prose styles for parser output, no parser dep
+├── chat-message-caret.tsx       # optional zero-width trailing caret
+├── lib/get-chat-date-time.ts    # Date | string → dateTime attribute
 ├── chat-message-layout.tsx      # ChatMessageContent: + streaming → data-streaming
 ├── chat-message.tsx             # + streaming, forwarded to content
 └── chat-message-text.tsx        # + children rendered after the text, + data-slot

<ChatMessage
  side="incoming"
  streaming={streaming}
  footer={<ChatMessageTimestamp dateTime={createdAt}>{label}</ChatMessageTimestamp>}
>
  <ChatMessageText text={text}>{streaming && <ChatMessageCaret />}</ChatMessageText>
</ChatMessage>

<ChatMessageMarkdown>
  <Markdown remarkPlugins={[remarkGfm]}>{text}</Markdown>  {/* host's parser */}
</ChatMessageMarkdown>

• The timestamp doesn't format anything. The host formats the label.
• The Markdown part only styles what it's given: headings, lists, links, inline and fenced code, tables, block quotes and rules. It uses chat's text-sm leading-[1.55]. Tables keep their table semantics, and a wrapper the parser adds around them scrolls.
• The streaming parts announce nothing. The docs point to live="off" plus the agent announcer (ticket 05).
• Three particles (p-chat-1 to p-chat-3) cover timestamp, Markdown and streaming. They're embedded in a new "Timestamps, Markdown, and streaming" section of chat.mdx.
• A new chat particle category.
• validate-registry-deps now maps @/registry/default/chat/* imports to @coss/chat, so particles can depend on the family.

Evidence

• Before: chat had no timestamp, Markdown or streaming parts (Cal used its own AgentMessageTimestamp and AGENT_MESSAGE_PROSE_CLASS_NAME).
  After:
  • bun test: 58 pass. That includes the new get-chat-date-time.test.ts cases:
    • Date → "2026-10-17T08:42:00.000Z"
    • strings pass through unchanged
    • an invalid Date gives undefined instead of throwing a RangeError
  • bun run typecheck is clean. registry:validate-deps is clean.
• Layout jump, measured in the docs preview while p-chat-3 streams: the caret's width is 0. Hiding the caret changes the text block's height by 0 and its width by 0. data-streaming="true" is set while streaming and removed afterwards.
• Reduced motion: the caret's pulse rule only exists inside @media (prefers-reduced-motion: no-preference).
• Tables: a <table> in p-chat-2 computes to display: table, and the overflow is on the container.

Merge Danger

Door: two-way

All changes are additive. The existing props on ChatMessage, ChatMessageContent and ChatMessageText are unchanged; the new ones are optional.

Blast Radius: small

• ChatMessageText now has data-slot="chat-message-text" and renders children after the text, so consumer CSS that matches [data-slot] could now also match it.
• The particles page gets a chat filter.

Not in this PR: the base branch has unrelated Biome drift (an unused ComposerQuery import in composer-editor.ts and class order in composer.tsx), and some public/r/p-*.json files are stale. I reverted both from this diff.

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[pasqualevitiello]

Closing: the Agent UI is a sibling family to Chat UI built on coss primitives, not on chat parts, so these chat additions aren't needed. The branch is kept.