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>

LOTUS_TESTING.md

@@ -677,8 +677,9 @@ Run the axe DevTools extension (or Lighthouse → Accessibility) on a room view,
 
 **Unread dot on federated rooms + avatar-decoration console storm (2026-07):**
 
-- Open a room from another homeserver that has thread activity; read it → the room's unread **dot clears** (previously an unread _thread reply_ kept the dot because `markAsRead` only sent an unthreaded receipt at the main-timeline tail). Also confirm opening a thread + reading it clears its part of the badge.
-- With DevTools console open on those rooms, the `io.lotus.avatar_decoration` `403`/`502` (and federated media) errors should **not** repeat on every scroll/mount — each failing user is now requested at most ~twice per session, so the storm (and its homeserver load) is gone.
+- **Read receipts (regression guard — highest priority):** open several rooms and open the Home/Direct tabs (which mark all orphan rooms read on mount) → rooms **stay read**, unread dots clear and don't come back. (A prior attempt sent a receipt for the thread _root_ when a thread's replies weren't loaded, which the SDK treats as a main receipt at an old event and re-unread every room on every mark-read. Fixed + locked by `notifications.test.ts`.)
+- **Thread dot:** a room with an unread reply in a thread whose replies are loaded → its dot clears on read; for a thread not yet loaded, the dot clears once you open/load the thread. (mark-as-read now sends a threaded receipt only for a genuine loaded reply, never the root.)
+- With DevTools console open on federated rooms, the `io.lotus.avatar_decoration` `403`/`502` (and federated media) errors should **not** repeat on every scroll/mount — each failing user is now requested at most ~twice per session, so the storm (and its homeserver load) is gone.
 
 **Custom Window Chrome (Beta) fix (2026-07):** on the desktop build, Settings → General → toggle **Custom Window Chrome** — it should reload and come up with the Lotus title bar and a normal, stable feed (no screen-expand / auto-scroll-into-the-past). Toggle back off → reloads to the native frame.