fix(timeline): thread panel shows read receipts; receipts recompute incrementally

- ReadPositionsContext is provided once at Room level so the thread
  panel (a sibling of RoomView) gets real positions instead of the empty
  default; own thread messages no longer sit on "Sent" forever (#38).
- Receipt events only recompute the users they name, merged into the
  previous map with reference equality preserved for untouched rows, so
  a receipt no longer re-renders every message (#40). Unit-tested.

Fixes #38
Fixes #40

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/features/room/RoomTimeline.tsx

@@ -90,8 +90,6 @@ import { useSetting } from '../../state/hooks/settings';
 import { MessageLayout, settingsAtom } from '../../state/settings';
 import { useMatrixEventRenderer } from '../../hooks/useMatrixEventRenderer';
 import { Reactions, Message, Event, EncryptedContent } from './message';
-import { ReadPositionsContext } from './ReadPositionsContext';
-import { useRoomReadPositions } from '../../hooks/useRoomReadPositions';
 import { useMemberEventParser } from '../../hooks/useMemberEventParser';
 import * as customHtmlCss from '../../styles/CustomHtml.css';
 import { RoomIntro } from '../../components/room-intro';
@@ -463,7 +461,8 @@ export function RoomTimeline({ room, eventId, roomInputRef, editor }: RoomTimeli
   const [messageSpacing] = useSetting(settingsAtom, 'messageSpacing');
   const [legacyUsernameColor] = useSetting(settingsAtom, 'legacyUsernameColor');
   const direct = useIsDirectRoom();
-  const readPositions = useRoomReadPositions(room);
+  // Read positions are computed once in Room.tsx and provided via ReadPositionsContext
+  // so both RoomTimeline and ThreadTimeline consume the same value (Gitea #38).
   const [hideMembershipEvents] = useSetting(settingsAtom, 'hideMembershipEvents');
   const [hideNickAvatarEvents] = useSetting(settingsAtom, 'hideNickAvatarEvents');
   const [mediaAutoLoad] = useSetting(settingsAtom, 'mediaAutoLoad');
@@ -2128,7 +2127,7 @@ export function RoomTimeline({ room, eventId, roomInputRef, editor }: RoomTimeli
   };
 
   return (
-    <ReadPositionsContext.Provider value={readPositions}>
+    <>
       <Box grow="Yes" style={{ position: 'relative' }}>
         {unreadInfo?.readUptoEventId && !unreadInfo?.inLiveTimeline && (
           <TimelineFloat position="Top">
@@ -2264,6 +2263,6 @@ export function RoomTimeline({ room, eventId, roomInputRef, editor }: RoomTimeli
           onClose={() => setEditHistoryEvent(undefined)}
         />
       )}
-    </ReadPositionsContext.Provider>
+    </>
   );
 }