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/__tests__/streamingMarkdown.test.ts

@@ -0,0 +1,79 @@
+import { describe, expect, it } from 'vitest'
+
+import { findStableBoundary } from '../components/streamingMarkdown.js'
+// We test the pure boundary logic by rendering the component's ref
+// behaviour through repeated calls. Since React isn't being rendered here,
+// we reach into the module to test findStableBoundary via its exported
+// behaviour — but the pure helper isn't exported. So test the component's
+// observable output: pass sequential text values and verify the stable
+// prefix never retreats.
+//
+// Strategy: mount StreamingMd in isolation and observe which <Md>
+// instances it renders (by text prop). Without a DOM renderer that's
+// heavy, so we validate the helper behaviour by directly invoking the
+// fence/boundary logic via a re-exported surface.
+import { DEFAULT_THEME } from '../theme.js'
+
+describe('findStableBoundary', () => {
+  it('returns -1 when no blank line exists yet', () => {
+    expect(findStableBoundary('partial line with no newline yet')).toBe(-1)
+  })
+
+  it('returns -1 when only single newlines exist', () => {
+    expect(findStableBoundary('line one\nline two\nline three')).toBe(-1)
+  })
+
+  it('splits after the last blank line separator', () => {
+    // 'first\n\nsecond\n\nthird' → last blank = before 'third'
+    const text = 'first paragraph\n\nsecond paragraph\n\nthird'
+    const idx = findStableBoundary(text)
+
+    expect(text.slice(0, idx)).toBe('first paragraph\n\nsecond paragraph\n\n')
+    expect(text.slice(idx)).toBe('third')
+  })
+
+  it('refuses to split inside an open fenced block', () => {
+    // Fence opens, contains a blank line inside the code, no close yet.
+    const text = '```ts\nfn();\n\nmore code here'
+
+    expect(findStableBoundary(text)).toBe(-1)
+  })
+
+  it('splits before an open fenced block but not inside', () => {
+    const text = 'intro paragraph\n\n```ts\nfn();\n\nmore code'
+    const idx = findStableBoundary(text)
+
+    expect(text.slice(0, idx)).toBe('intro paragraph\n\n')
+    expect(text.slice(idx).startsWith('```ts')).toBe(true)
+  })
+
+  it('allows splitting after a fenced block closes', () => {
+    const text = '```ts\nfn();\n```\n\nnarration continues'
+    const idx = findStableBoundary(text)
+
+    expect(text.slice(0, idx)).toBe('```ts\nfn();\n```\n\n')
+    expect(text.slice(idx)).toBe('narration continues')
+  })
+
+  it('walks backwards through nested fence boundaries safely', () => {
+    // Two closed fences + narration + one new open fence. The only legal
+    // split is before the open fence, not between the closed ones.
+    const text = '```js\na\n```\n\nmid text\n\n```python\nstill open'
+    const idx = findStableBoundary(text)
+
+    expect(text.slice(0, idx)).toBe('```js\na\n```\n\nmid text\n\n')
+  })
+
+  it('handles empty input', () => {
+    expect(findStableBoundary('')).toBe(-1)
+  })
+})
+
+describe('streaming theme assumption', () => {
+  it('theme is exportable (component import sanity check)', () => {
+    // Sanity that the theme we pass doesn't change shape. Component import
+    // already happens above — this is a smoke test that the module graph
+    // for streamingMarkdown wires up without cycles.
+    expect(DEFAULT_THEME.color.amber).toBeTruthy()
+  })
+})