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/app/turnController.ts

@@ -182,6 +182,18 @@ class TurnController {
     }, REASONING_PULSE_MS)
   }
 
+  /**
+   * Append an inline artifact (e.g. tool-complete inline diff) to the active
+   * turn's segment stream. Routing through `historyItems` via `sys()` lands
+   * the artifact above the currently-streaming assistant bubble; adding it
+   * here keeps the paint order aligned with the order the gateway emitted.
+   */
+  appendSegmentMessage(msg: Msg) {
+    this.flushStreamingSegment()
+    this.segmentMessages = [...this.segmentMessages, msg]
+    patchTurnState({ streamSegments: this.segmentMessages })
+  }
+
   pushActivity(text: string, tone: ActivityItem['tone'] = 'info', replaceLabel?: string) {
     patchTurnState(state => {
       const base = replaceLabel