perf: isolate chat streaming renders and reduce sidebar render cost (#1672)

Reworks the chat and session-sidebar render paths to cut render cascades, memory
  churn, and UI jank on large sessions and big session trees. Behavior is preserved;
  the changes are about *when* and *how much* the UI re-renders.

  ## Chat streaming
  - Freeze the streaming message's parts in the bulk turn projection during streaming,
    and re-inject live parts only in an isolated tail leaf, so a ~60/sec delta stream
    no longer re-runs the whole-session projection or re-renders unrelated rows.
    session with referential reuse of unchanged turns.
  - Memoize message rows with field-aware comparators instead of reference equality.
  - Replace the manual child-session polling in the task tool with the live SSE
    stream + a one-shot load, removing a fetch/settle state machine.

  ## History loading & scroll
  - Load an initial page fast, then prepend one older page in the background so the
    scroll container has headroom and "load older on scroll-up" fires before the user
    hits the absolute top.
  - Compensate scroll synchronously (in a layout effect, before paint) for prepends —
    including background prepends that don't originate from a user scroll — so the
    viewport stays stable instead of judder-correcting on the next frame.

  ## Markdown rendering
  - Render markdown synchronously *styled* on first paint (paragraphs, lists, code
    cards, tables, inline code) instead of raw escaped text; the async pass then only
    upgrades syntax-highlight colors. Eliminates the flash of full-width raw text.
  - Load KaTeX CSS eagerly with the main bundle instead of inside the lazy markdown
    chunk, avoiding a late stylesheet injection on first render.

  ## Sidebar
  - Hoist per-row recursive tree walks out of row comparators into per-group
    precomputed sets/keys; batch live-session lookups into a single map; add a
    group-level memo boundary.
  - Isolate rename drafts so per-keystroke typing doesn't repaint the row tree.

  ## Sync layer
  - Add a staleness guard so a slow message fetch can't repopulate a session the user
    navigated away from.
  - Throw on fetch failure for authoritative loaders so a transient blip can't read as
    an empty server response.

  ## Cleanup
  - Remove dead code (unused hooks, params, duplicated inline types) surfaced while
    reworking the above.

  ## Known issue
  - A rare, purely cosmetic first-paint width flash can still appear on large sessions;
    it has no behavioral or data impact and is tracked for a follow-up runtime trace.

packages/ui/src/components/chat/ChatInput.tsx

@@ -74,6 +74,7 @@ import { sessionEvents } from '@/lib/sessionEvents';
 import { fetchResponseStyleInstruction } from '@/lib/responseStyle';
 import { wrapSystemReminder } from '@/lib/systemReminder';
 import { getSyncMessages } from '@/sync/sync-refs';
+import { EMPTY_REVERTED_MESSAGE_DOCK_STATE, buildRevertedMessageDockState, type RevertedMessageDockState } from './revertedMessageDockState';
 import { eventMatchesShortcut, getEffectiveShortcutCombo, normalizeCombo } from '@/lib/shortcuts';
 import { isSyntheticPart } from '@/lib/messages/synthetic';
 import {
@@ -89,11 +90,10 @@ import {
     buildAttachmentCitationText,
     findAttachmentCitationRanges,
 } from './attachmentCitations';
-import type { Message, Part } from '@opencode-ai/sdk/v2/client';
+import type { Part } from '@opencode-ai/sdk/v2/client';
 
 const MAX_VISIBLE_TEXTAREA_LINES = 8;
 const EMPTY_QUEUE: QueuedMessage[] = [];
-const EMPTY_MESSAGES: Message[] = [];
 const FILE_MENTION_TOKEN = /^@[^\s]+$/;
 // Single-line URL pasted over a selection becomes a markdown link.
 const PASTE_LINK_URL_PATTERN = /^(https?:\/\/|mailto:)\S+$/i;
@@ -363,34 +363,28 @@ const RevertedMessageDock: React.FC<RevertedMessageDockProps> = React.memo(({ se
     const [restoringId, setRestoringId] = React.useState<string | null>(null);
     const [forkingId, setForkingId] = React.useState<string | null>(null);
     const [collapsed, setCollapsed] = React.useState(true);
-    const revertMessageID = useDirectorySync(
+    const revertedStateRef = React.useRef<RevertedMessageDockState>(EMPTY_REVERTED_MESSAGE_DOCK_STATE);
+    const revertedState = useDirectorySync(
         React.useCallback((state) => {
-            if (!sessionId) return undefined;
-            const session = state.session.find((item) => item.id === sessionId);
-            return (session as { revert?: { messageID?: string } } | undefined)?.revert?.messageID;
+            const next = buildRevertedMessageDockState(state, sessionId, revertedStateRef.current);
+            revertedStateRef.current = next;
+            return next;
         }, [sessionId]),
         directory,
     );
-    const sessionMessages = useDirectorySync(
-        React.useCallback((state) => (sessionId ? state.message[sessionId] ?? EMPTY_MESSAGES : EMPTY_MESSAGES), [sessionId]),
-        directory,
-    );
-    const partsByMessage = useDirectorySync(React.useCallback((state) => state.part, []), directory);
-
+    const revertMessageID = revertedState.revertMessageID;
     const userMessages = React.useMemo(
-        () => sessionMessages.filter((message): message is Message & { role: 'user' } => message.role === 'user'),
-        [sessionMessages],
+        () => revertedState.records.map((record) => record.message),
+        [revertedState],
     );
     const noTextContent = t('chat.revertPopover.noTextContent');
     const items = React.useMemo(() => {
         if (!revertMessageID) return [];
-        return userMessages
-            .filter((message) => message.id >= revertMessageID)
-            .map((message) => ({
-                id: message.id,
-                text: getRevertedPreview(partsByMessage[message.id] ?? [], noTextContent),
-            }));
-    }, [noTextContent, partsByMessage, revertMessageID, userMessages]);
+        return revertedState.records.map((record) => ({
+            id: record.message.id,
+            text: getRevertedPreview(record.parts, noTextContent),
+        }));
+    }, [noTextContent, revertMessageID, revertedState]);
     const firstRevertedMessageId = items[0]?.id;
 
     React.useEffect(() => {