fix: address review — plain fences while streaming

ds-review-bot finding: a growing fence retokenized on every chunk
(quadratic main-thread work). MarkdownText gains a streaming flag —
the streaming partial renders fences through the plain arm and the
finalize swap highlights once; AssistantMarkdown threads its existing
flag. (The zh Agent Note pair the review also flagged landed earlier
on this branch.) New spec pins plain-while-streaming and
highlighted-after-finalize.

packages/client/ui-conversation/src/client/chat/AssistantMarkdown.tsx

@@ -44,7 +44,7 @@ export const AssistantMarkdown = memo(function AssistantMarkdown({ blocks, strea
     <div className={css.root} data-streaming={streaming || undefined}>
       {blocks.map((block, i) => {
         switch (block.kind) {
-          case 'text': return <MarkdownText key={i} text={block.text} />
+          case 'text': return <MarkdownText key={i} text={block.text} streaming={streaming} />
           case 'reasoning': return <ThinkRow key={i} text={block.text} running={streaming && i === last} />
           // Tool-call heads render as tool rows in the chat view's grouping pass.
           case 'tool-call': return null