fix(tui): tool inline_diff renders inline with the active turn

Reported during TUI v2 blitz retest: code-review diffs from tool.complete
appeared at the top of the current interaction thread, out of sequence
with the agent's messages and tool rows below them.

Root cause — `sys(inline_diff)` appends to `historyItems`, which sits
above the `StreamingAssistant` pane that renders the active turn.
Until the turn closed, the diff visually floated above everything
else happening in the same turn.

Route the diff through `turnController.appendSegmentMessage` instead
so it flushes any pending streaming text first, then lands in the
segment stream beside assistant output and tool calls.  On
`message.complete` the segment list is committed to history in emit
order (diff → final text), matching what the gateway sent.

Adds a regression test that exercises tool.complete → message.complete
with an inline_diff payload and asserts both the streaming and final
placement.

ui-tui/src/__tests__/createGatewayEventHandler.test.ts

@@ -143,6 +143,38 @@ describe('createGatewayEventHandler', () => {
     expect(appended[0]?.thinkingTokens).toBe(estimateTokensRough(fromServer))
   })
 
+  it('routes inline_diff into the active segment stream, not historyItems', () => {
+    const appended: Msg[] = []
+    const onEvent = createGatewayEventHandler(buildCtx(appended))
+    const diff = '--- a/foo.ts\n+++ b/foo.ts\n@@\n-old\n+new'
+
+    onEvent({
+      payload: { context: 'foo.ts', name: 'patch', tool_id: 'tool-1' },
+      type: 'tool.start'
+    } as any)
+    onEvent({
+      payload: { inline_diff: diff, summary: 'patched', tool_id: 'tool-1' },
+      type: 'tool.complete'
+    } as any)
+
+    // While streaming, nothing has flowed to historyItems yet — diff must be
+    // held in segmentMessages so the transcript renders it inline with the
+    // current turn rather than above it.
+    expect(appended).toHaveLength(0)
+    expect(turnController.segmentMessages).toContainEqual({ role: 'system', text: diff })
+
+    onEvent({
+      payload: { text: 'patch applied' },
+      type: 'message.complete'
+    } as any)
+
+    // After the turn closes, the diff lands in history in the order the
+    // gateway emitted it — before the assistant's final text, not above it.
+    expect(appended).toHaveLength(2)
+    expect(appended[0]).toMatchObject({ role: 'system', text: diff })
+    expect(appended[1]).toMatchObject({ role: 'assistant', text: 'patch applied' })
+  })
+
   it('shows setup panel for missing provider startup error', () => {
     const appended: Msg[] = []
     const onEvent = createGatewayEventHandler(buildCtx(appended))