docs: Slack-style per-thread notifications (P4-1) across catalog/README/TODO/BUGS

LOTUS_FEATURES: Notifications subsection under Threads (participating default,
per-thread All/Mentions/Mute, badge behavior). README: thread-notifications
bullet. LOTUS_TODO: P4-1 → [~] + 6-step live-QA checklist + caveats.
LOTUS_BUGS: verification row.

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

LOTUS_FEATURES.md

@@ -800,9 +800,14 @@ Root messages in the main timeline show a **"N replies · time"** chip (server-a
 
 The panel embeds the full composer (uploads, emoji, stickers, GIFs, voice, location, polls) with drafts, reply state, and upload queues **isolated per thread** (`roomId::threadRootId` keys). Replies-to-replies produce spec-correct `m.thread` + `m.in_reply_to` (`is_falling_back: false`). Scheduling and slash commands are disabled inside threads (v1).
 
+### Notifications (Slack-style, P4-1)
+
+By default you're notified for a thread reply only when you **participate** in that thread (you've posted in it) or the reply **@mentions** you — other threads accumulate quietly behind their chip badges. Every thread can be overridden from the bell menu in the panel header: **Default (participating) / All replies / Mentions only / Mute**. Modes sync across your devices (`io.lotus.thread_notifications` account data, auto-pruned). Muting a thread silences notifications and sounds, removes the chip's unread badge (a small bell-mute glyph shows instead), and subtracts that thread from the room's sidebar unread badge (client-side — other Matrix clients on the account still count it).
+
 ### Under the Hood
 
 - `threadSupport: true` (startClient) partitions thread events into SDK `Thread` timelines; markAsRead sends **unthreaded** receipts so room badges keep clearing
+- Thread replies are notified via exactly one path (room-level `ThreadEvent.NewReply` w/ per-thread dedupe + panel-aware focus suppression); the main timeline notifier is thread-guarded, and room badges refresh live on `RoomEvent.UnreadNotifications`
 - Pending sends render via a `LocalEchoUpdated` strip (chronological local echo never enters thread timelineSets)
 - Deep links to thread events redirect into the panel
 - Files: `features/room/thread/*`, `state/room/thread.ts`, `hooks/useThreadSummary.ts` (+35 tests across the stack)