refactor(desktop): tighten reasoning part typing, drop dead useRef

Self-review nits on the Thinking-widget fix:
- type ReasoningTextPart as ReasoningMessagePartComponent and read the
  typed useMessagePartReasoning() directly, dropping the ad-hoc cast
  (the hook already returns text/status).
- remove useRef, now unused after deleting useSmoothReveal.
- trim the autopsy comments; the PR body carries the narrative.

apps/desktop/src/components/assistant-ui/markdown-text.tsx

@@ -15,7 +15,6 @@ import {
   useDeferredValue,
   useEffect,
   useMemo,
-  useRef,
   useState
 } from 'react'
 
@@ -588,12 +587,9 @@ interface MarkdownTextContentProps extends MarkdownTextSurfaceProps {
 }
 
 export function MarkdownTextContent({ isRunning, text, ...surfaceProps }: MarkdownTextContentProps) {
-  // Render through the same path as the assistant answer (DeferStreamingText →
-  // surface). The previous SmoothStreamingText/useSmoothReveal wrapper stalled
-  // at empty for reasoning: the reasoning part stays isRunning for the whole
-  // message while the answer streams and thrashes re-renders, and the reveal
-  // never advanced past 0 — so the Thinking widget rendered blank even though
-  // the part carried text. Chunked (per-delta) reveal matches the answer.
+  // Same path as the assistant answer. A reasoning-only smoothing wrapper used
+  // to sit here but stalled its char-reveal at empty (the part stays running
+  // the whole message), blanking the Thinking widget.
   return (
     <TextMessagePartProvider isRunning={isRunning} text={text}>
       <DeferStreamingText>

apps/desktop/src/components/assistant-ui/thread/message-parts.tsx

@@ -1,4 +1,9 @@
-import { type ToolCallMessagePartProps, useAuiState, useMessagePartReasoning } from '@assistant-ui/react'
+import {
+  type ReasoningMessagePartComponent,
+  type ToolCallMessagePartProps,
+  useAuiState,
+  useMessagePartReasoning
+} from '@assistant-ui/react'
 import { type ComponentProps, type FC, type ReactNode, useEffect, useRef, useState } from 'react'
 
 import { ClarifyTool } from '@/components/assistant-ui/clarify-tool'
@@ -174,23 +179,19 @@ const ReasoningAccordionGroup: FC<{ children?: ReactNode; endIndex: number; star
   )
 }
 
-const ReasoningTextPart: FC = () => {
-  // Read the reasoning text from the assistant-ui part context (same contract
-  // as MarkdownText's useMessagePartText). The `Reasoning` component is NOT
-  // handed a `text` prop — the runtime provides the part via context — so the
-  // previous prop-based read rendered an empty string and the Thinking widget
-  // showed blank even though the part carried text.
-  const part = useMessagePartReasoning() as { text?: string; status?: { type?: string } }
+// Read the part from context, same contract as MarkdownText's
+// useMessagePartText — the reasoning-only smoothing wrapper (removed) stalled
+// the char-reveal at empty, blanking the widget.
+const ReasoningTextPart: ReasoningMessagePartComponent = () => {
+  const { status, text } = useMessagePartReasoning()
   const messageRunning = useAuiState(s => s.message.status?.type === 'running')
-  const isRunning = part.status?.type === 'running' || messageRunning
-  const displayText = (part.text ?? '').trimStart()
 
   return (
     <MarkdownTextContent
       containerClassName="text-xs leading-snug text-muted-foreground/85"
       containerProps={{ 'data-slot': 'aui_reasoning-text' } as ComponentProps<'div'>}
-      isRunning={isRunning}
-      text={displayText}
+      isRunning={status.type === 'running' || messageRunning}
+      text={text.trimStart()}
     />
   )
 }