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/createGatewayEventHandler.ts

@@ -266,7 +266,12 @@ export function createGatewayEventHandler(ctx: GatewayEventHandlerContext): (ev:
         turnController.recordToolComplete(ev.payload.tool_id, ev.payload.name, ev.payload.error, ev.payload.summary)
 
         if (ev.payload.inline_diff && getUiState().inlineDiffs) {
-          sys(ev.payload.inline_diff)
+          // Push into the active turn's segment stream so the diff renders
+          // inline with the assistant's output.  Routing through `sys()`
+          // lands it in the completed-history section above the streaming
+          // bubble — which is why blitz testers saw diffs "appear at the
+          // top, out of sequence" with the rest of the turn.
+          turnController.appendSegmentMessage({ role: 'system', text: ev.payload.inline_diff })
         }
 
         return