feat(messaging-ui): channels — browse, join, leave, create (contract + mock + UI)

Adds a third membership beyond dm/group: a discoverable, joinable room.

- contract: Membership += 'channel'; Conversation.topic; ChannelSummary +
  CreateChannelInput; optional adapter methods browseChannels/createChannel/
  joinChannel/leaveChannel (capability-degrading — absent hides the UI)
- MockAdapter implements them (seeds a joined public, a joinable public, and a
  private channel); listConversations now returns only threads I'm in
- useChannels hook (browse/create/join/leave + supported flag)
- UI: sidebar sections (Channels vs Direct messages), a ＋ that opens a
  ChannelBrowser (join + create), # / lock glyphs; themeable styles
- KernelClientAdapter passes channel membership + topic through
- 4 channel tests (mock browse/join/create + render browse→join); 52 total green

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

packages/iios-messaging-ui/src/index.ts

@@ -6,19 +6,25 @@
 export { MessagingProvider, useAdapter } from './provider';
 export { useConversations } from './hooks/use-conversations';
 export { useMessages } from './hooks/use-messages';
+export { useChannels } from './hooks/use-channels';
 export { isOwnMessage } from './types';
 
 // Rendered UI. Pair with the './styles.css' export (or override the --miu-* tokens).
 export { Messenger } from './components/messenger';
 export { ConversationList } from './components/conversation-list';
+export { ChannelBrowser } from './components/channel-browser';
 export { Thread } from './components/thread';
 
 export type { MessagingAdapter } from './adapter';
 export type { ConversationsState } from './hooks/use-conversations';
 export type { MessagesState, UiMessage } from './hooks/use-messages';
+export type { ChannelsState } from './hooks/use-channels';
 export type {
   Attachment,
+  ChannelSummary,
+  ChannelVisibility,
   Conversation,
+  CreateChannelInput,
   Membership,
   Message,
   MessageEvent,