fix(notifications): safe thread receipts on mark-read (fixes read-receipt regression)

The prior thread-receipt change (8192da5a) broke read receipts globally. Exact
cause: markAsRead used `thread.lastReply() ?? thread.rootEvent`. When a thread's
replies weren't loaded (lastReply() null — common on room open), it sent a
receipt for the thread ROOT. Since roots are "in the main timeline",
threadIdForReceipt() makes that a MAIN receipt at an old event; when the root
isn't in the loaded timeline the SDK's backward-guard falls back to timestamp
and applies it, moving the main read receipt onto an event we don't have, so
getEventReadUpTo() returns null and roomHaveUnread() reports the room unread —
re-broken on every mark-read, amplified by the bulk mark-all-orphan-rooms-read
callers.

Fix: main unthreaded receipt unchanged; the thread loop now sends a threaded
receipt ONLY for a genuine loaded thread reply (thread.lastReply()), never the
root — if replies aren't loaded, skip. New notifications.test.ts locks the
regression (null lastReply → no root receipt) + the main/threaded/no-op cases.

Gates: tsc/eslint/prettier clean, build OK, 672 tests (7 new).

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

src/app/utils/notifications.ts

@@ -25,25 +25,33 @@ export async function markAsRead(mx: MatrixClient, roomId: string, privateReceip
   if (latestEvent) {
     // Unthreaded receipt: with client threadSupport enabled the SDK would
     // otherwise scope this to the main timeline (thread_id: "main"). Unthreaded
-    // clears the main timeline + every event up to this one, in any thread.
+    // clears the main timeline + every event up to this one.
     await mx.sendReadReceipt(latestEvent, receiptType, true);
   }
 
-  // ...but a thread reply NEWER than the main-timeline tail is not covered by
-  // the receipt above (threadSupport moves thread replies out of the main
-  // timeline), so its per-thread notification count — and the room's unread dot,
-  // which sums thread counts — would linger even after "reading" the room. Send
-  // a threaded receipt at the latest reply of every thread that still has unread
-  // counts. Also runs when the main timeline is already read (latestEvent null).
+  // Clear per-thread notification counts too — the room's unread dot sums them,
+  // so an unread thread reply keeps the dot lit even after the main timeline is
+  // read (threadSupport moves thread replies out of the main timeline, so the
+  // unthreaded receipt above doesn't necessarily cover them).
+  //
+  // CRITICAL: only send for a GENUINE loaded thread reply, via thread.lastReply().
+  // NEVER fall back to the thread root: a root event is "in the main timeline",
+  // so sendReadReceipt(root, false) resolves (via threadIdForReceipt) to a MAIN
+  // receipt at that old root event. If the root isn't in the loaded timeline it
+  // moves the main read receipt onto an event we don't have -> getEventReadUpTo()
+  // returns null -> the room is reported unread on every mark-read call (this was
+  // the P6 regression, amplified by the bulk mark-all-orphan-rooms-read callers).
+  // If a thread's replies aren't loaded (lastReply() null), just skip it.
   const threads = room.getThreads();
   await Promise.all(
     threads.map((thread) => {
       const unread =
         room.getThreadUnreadNotificationCount(thread.id, NotificationCountType.Total) ?? 0;
       if (unread <= 0) return undefined;
-      const lastReply = thread.lastReply() ?? thread.rootEvent;
+      const lastReply = thread.lastReply();
       if (!lastReply || lastReply.isSending()) return undefined;
-      // Threaded receipt (unthreaded = false → the SDK scopes it to this thread).
+      // Threaded receipt (unthreaded = false → the SDK scopes it to this thread
+      // via the reply's real threadRootId; it never touches the main marker).
       return mx.sendReadReceipt(lastReply, receiptType, false).catch(() => undefined);
     }),
   );