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/MarkdownRendererImpl.tsx

@@ -1,5 +1,4 @@
 import React from 'react';
-import 'katex/dist/katex.min.css';
 import morphdom from 'morphdom';
 import { renderMermaidASCII, renderMermaidSVG } from 'beautiful-mermaid';
 import type { Part } from '@opencode-ai/sdk/v2';
@@ -19,7 +18,7 @@ import type { EditorAPI } from '@/lib/api/types';
 import { isDesktopLocalOriginActive, isDesktopShell, isVSCodeRuntime } from '@/lib/desktop';
 import { ensureOutsideFileGrantForDesktop } from '@/lib/outsideFileGrants';
 import { getDirectoryForFilePath, isAbsoluteFilePath, isFilePathWithinDirectory, normalizeFilePath, toAbsoluteFilePath } from '@/lib/path-utils';
-import { fallbackHtml, renderMarkdownBlocks } from './markdown/markdownCore';
+import { renderMarkdownBlocks, renderMarkdownSync } from './markdown/markdownCore';
 import { ensureMarkdownShikiTheme, getMarkdownSyntaxVars } from './markdown/markdownTheme';
 import {
   attachMarkdownInteractions,
@@ -1032,10 +1031,16 @@ const useMorphdomMarkdown = ({
       // `display:contents` keeps margin-collapsing/spacing identical to a flat
       // HTML body — the wrapper exists only for per-block reconciliation.
       block.style.display = 'contents';
-      block.innerHTML = fallbackHtml(text);
+      block.innerHTML = renderMarkdownSync(text);
+      // Decorate synchronously too: wrap code blocks in their framed card,
+      // mark inline code, build table controls, etc. The async pass re-decorates
+      // its own DOM before morphing, so without this the first paint shows bare
+      // <pre>/tables that "snap" into their decorated form a tick later. Matching
+      // the structure here keeps the async morph to syntax colors only.
+      decorateMarkdown(block, ctx);
       target.appendChild(block);
     }
-  }, [containerRef, text]);
+  }, [containerRef, text, ctx]);
 
   React.useEffect(() => {
     const container = containerRef.current;