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/sync/session-ui-store.ts

@@ -51,6 +51,7 @@ import {
   revertToMessage as revertToMessageAction,
   unrevertSession as unrevertSessionAction,
   forkFromMessage as forkFromMessageAction,
+  fetchMessagesForSession,
 } from "./session-actions"
 import { useInputStore, type SyntheticContextPart } from "./input-store"
 import { useSelectionStore } from "./selection-store"
@@ -497,6 +498,13 @@ export const useSessionUIStore = create<SessionUIState>()((set, get) => ({
     set({ currentSessionId: id, currentSessionDirectory: id ? resolvedDir ?? null : null })
     writeRuntimeSessionMemory(key, { sessionId: id, directory: resolvedDir ?? null })
 
+    // Kick off the message fetch on the same tick, before React commits the
+    // state change and fires ChatContainer.useEffect. The fetch is
+    // fire-and-forget — any transient failure gets retried by the reactive path.
+    if (id) {
+      void fetchMessagesForSession(id, resolvedDir)
+    }
+
     try {
       if (resolvedDir && directoryState.currentDirectory !== resolvedDir) {
         directoryState.setDirectory(resolvedDir, { showOverlay: false })