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} />
       )

ui-tui/src/components/streamingMarkdown.tsx

@@ -0,0 +1,127 @@
+// StreamingMd — incremental markdown renderer for in-flight assistant text.
+//
+// Naive approach (render <Md text={full}/>) re-tokenizes the entire message
+// on every stream delta. At 20-char batches over a 3 KB response that's 150
+// full re-parses.
+//
+// This splits `text` at the last stable top-level block boundary (blank
+// line outside a fenced code span) into:
+//   stablePrefix — passed to an inner <Md>, memoized on its exact text
+//                  value. During the turn, the prefix only grows monotonically,
+//                  so its memo key matches the previous render and React
+//                  reuses the cached subtree — zero re-tokenization.
+//   unstableSuffix — the in-flight block(s). A separate <Md> re-parses just
+//                    this tail on every delta (O(unstable length) vs.
+//                    O(total length)).
+//
+// The boundary is stored in a ref so it only advances — idempotent under
+// StrictMode double-render. Component unmounts between turns (isStreaming
+// flips off → message moves to history and renders via <Md> directly), so
+// the ref resets naturally.
+//
+// See src/app/useMainApp.ts for the reasoning on why we don't memoize the
+// whole Md text during streaming: that cache never hits because `text` is
+// growing. Mirror claude-code's `StreamingMarkdown` approach adapted to
+// our line-based tokenizer.
+
+import { memo, useRef } from 'react'
+
+import type { Theme } from '../theme.js'
+
+import { Md } from './markdown.js'
+
+// Count ``` or ~~~ fence toggles in `s` up to `end`. Odd = currently inside
+// a fenced block; we can't split the prefix there or we'd orphan the fence.
+const fenceOpenAt = (s: string, end: number) => {
+  let open = false
+  let i = 0
+
+  while (i < end) {
+    const nl = s.indexOf('\n', i)
+    const lineEnd = nl < 0 || nl > end ? end : nl
+    const line = s.slice(i, lineEnd)
+
+    if (/^\s*(?:`{3,}|~{3,})/.test(line)) {
+      open = !open
+    }
+
+    if (nl < 0 || nl >= end) {
+      break
+    }
+
+    i = nl + 1
+  }
+
+  return open
+}
+
+// Find the last "\n\n" boundary before `end` that is OUTSIDE a fenced code
+// block. Returns the index AFTER the second newline (start of the next
+// block), or -1 if no safe boundary exists yet.
+export const findStableBoundary = (text: string) => {
+  let idx = text.length
+
+  while (idx > 0) {
+    const boundary = text.lastIndexOf('\n\n', idx - 1)
+
+    if (boundary < 0) {
+      return -1
+    }
+
+    // Boundary candidate: end of stable prefix is boundary + 2 (start of
+    // next block). Check fence balance up to that point.
+    const splitAt = boundary + 2
+
+    if (!fenceOpenAt(text, splitAt)) {
+      return splitAt
+    }
+
+    idx = boundary
+  }
+
+  return -1
+}
+
+export const StreamingMd = memo(function StreamingMd({ compact, t, text }: StreamingMdProps) {
+  const stablePrefixRef = useRef('')
+
+  // Reset if the text no longer starts with our recorded prefix (defensive;
+  // normally the component unmounts between turns so this shouldn't trigger).
+  if (!text.startsWith(stablePrefixRef.current)) {
+    stablePrefixRef.current = ''
+  }
+
+  const boundary = findStableBoundary(text)
+
+  // Only advance the prefix — never retreat. The boundary math looks at the
+  // FULL text each call; if it returns a larger index than before, we grow
+  // the cached prefix. Monotonic growth makes the memo key stable across
+  // deltas (identical string → same <Md> subtree → no re-render).
+  if (boundary > stablePrefixRef.current.length) {
+    stablePrefixRef.current = text.slice(0, boundary)
+  }
+
+  const stablePrefix = stablePrefixRef.current
+  const unstableSuffix = text.slice(stablePrefix.length)
+
+  if (!stablePrefix) {
+    return <Md compact={compact} t={t} text={unstableSuffix} />
+  }
+
+  if (!unstableSuffix) {
+    return <Md compact={compact} t={t} text={stablePrefix} />
+  }
+
+  return (
+    <>
+      <Md compact={compact} t={t} text={stablePrefix} />
+      <Md compact={compact} t={t} text={unstableSuffix} />
+    </>
+  )
+})
+
+interface StreamingMdProps {
+  compact?: boolean
+  t: Theme
+  text: string
+}