Discussion Integration

Controlled state, DiscussionAdapter trait, and host wiring patterns.

• Connecting a discussion thread to server actions or a custom backend adapter.
• Verifying use_discussion hooks against controlled reply and focus signals.

• Hold replies in a controlled Signal<Vec<DiscussionReply>>.
• Implement DiscussionAdapter::submit_reply in your app (HTTP, server fn, etc.).
• Bridge composer submit via DiscussionEvents::on_submit — call the adapter asynchronously and push the returned reply into your signal.
• Optionally pass DiscussionAttachmentValidation and wire events.on_attachment_reject on DiscussionThread for client-side attach rules; re-validate attachments in DiscussionAdapter::submit_reply before persisting.

Only submit_reply is required. Override DiscussionAdapter::fetch_branch when your backend supports lazy branch loading (hidden_child_count, show-more drill-in). Override DiscussionAdapter::upload_attachment when the composer should persist attachment drafts to host storage before submit.