perf(tui): incremental markdown during streaming

Split in-flight assistant text at the last stable block boundary so only
the unclosed tail re-tokenizes per stream delta. Previously the full
text was rendered as plain <Text> during streaming and only flipped to
<Md> at message.complete — cheap per delta but loses live markdown
formatting.

New StreamingMd component holds a monotonically-growing stablePrefix
in a ref (idempotent under StrictMode double-render), renders it as
one <Md> that memoizes across deltas, and renders the unstable suffix
as a second <Md> that re-parses on each delta. Cost per delta drops
from O(total length) to O(unstable length).

findStableBoundary walks back to the last "\n\n" outside an open
fenced code block — splitting inside an open fence would orphan the
opener and break highlighting in the prefix.

Adapted from claude-code's src/components/Markdown.tsx:186 but built
on our line-based tokenizer instead of marked.lexer. 9 new tests cover
fence balance, boundary walk, and empty input.

Part of the --tui perf audit (see audit #7).

ui-tui/src/components/messageLine.tsx

@@ -10,6 +10,7 @@ import type { Theme } from '../theme.js'
 import type { ActiveTool, DetailsMode, Msg, SectionVisibility } from '../types.js'
 
 import { Md } from './markdown.js'
+import { StreamingMd } from './streamingMarkdown.js'
 import { ToolTrail } from './thinking.js'
 import { TodoPanel } from './todoPanel.js'
 
@@ -94,7 +95,10 @@ export const MessageLine = memo(function MessageLine({
 
     if (msg.role === 'assistant') {
       return isStreaming ? (
-        <Text color={body}>{boundedLiveRenderText(msg.text)}</Text>
+        // Incremental markdown: split at the last stable block boundary so
+        // only the in-flight tail re-tokenizes per delta. See
+        // streamingMarkdown.tsx for the cost model.
+        <StreamingMd compact={compact} t={t} text={boundedLiveRenderText(msg.text)} />
       ) : (
         <Md compact={compact} t={t} text={msg.text} />
       )