feat(chat): stop an in-flight answer — Send becomes Stop, the partial is kept and persisted, the model stream is torn down

tests/unit/test_frontend_scroll.py

@@ -193,7 +193,9 @@ def test_turn_end_focus_does_not_scroll() -> None:
     up would yank them to the bottom at the moment the turn ends.
     preventScroll keeps the keyboard flow without the scroll."""
     js = _js()
-    finally_idx = js.find("// done | error → idle: always settle, always focus back")
+    # Phase 48: the settle line gained the user-stop terminal (stop →
+    # idle, no banner) — the focus-back contract is unchanged.
+    finally_idx = js.find("// done | error | stop → idle: always settle, always focus back")
     assert finally_idx != -1, "the turn's finally block must exist"
     block = js[finally_idx : js.find("\n}", finally_idx)]
     assert 'input.focus({ preventScroll: true })' in block