chore: pin scene-close-on-cancel behavior + comment rationale (T74.3)

Phase 2 T44 review noted that scene close still runs when a primary
turn is cancelled mid-stream and asked the implementer to review.

Review finding: the existing behavior is correct, not a bug. The
close-detection branch in post_turn consumes ONLY the user's prose
(fully appended to the event_log BEFORE streaming starts) and the
current container name. It does NOT consume the bot's output. A user
who types "we're done here, fade out" and then hits Stop mid-stream
still meant to close — the cancelled bot beat doesn't invalidate
that intent.

- Document the rationale with an inline comment near the
  close-detection branch in chat/web/turns.py.
- Add regression test
  test_cancelled_turn_still_closes_scene_when_user_prose_signals_close
  that drives a stream raising CancelledError on first iteration and
  asserts the scene_closed event still lands.

chat/web/turns.py

@@ -668,6 +668,15 @@ async def post_turn(
     # close in the same chat) — we have nothing to close. T13 (kickoff)
     # is the only scene-opener path in v1; Phase 2-3 will handle
     # automatic re-opening with the next container.
+    #
+    # T74.3: this branch deliberately runs even when ``cancelled`` is
+    # True. Close detection consumes only the user's prose (which is
+    # fully appended to the event_log BEFORE streaming starts) and the
+    # current container name; it does NOT consume the bot's output.
+    # A user who types "we're done here, fade out" and then hits Stop
+    # mid-stream still meant to close the scene — the cancelled bot
+    # beat doesn't invalidate that intent. Pinned by
+    # test_cancelled_turn_still_closes_scene_when_user_prose_signals_close.
     if scene is not None and prose.strip():
         container = None
         if scene.get("container_id") is not None:

tests/test_turn_flow.py

@@ -715,6 +715,127 @@ def test_addressee_detection_routes_to_named_bot(app_state_setup, tmp_path):
     assert interjection_payload["interjection_of"] == "bot_b"
 
 
+def test_cancelled_turn_still_closes_scene_when_user_prose_signals_close(
+    app_state_setup, tmp_path
+):
+    """T74.3 regression: a cancelled primary stream still triggers scene
+    close when the user prose carries a hard close signal.
+
+    Rationale (also documented in turns.py near the close-detection
+    branch): close detection only consumes the user's prose, which is
+    fully appended to the event_log BEFORE streaming starts. The
+    cancelled bot beat doesn't invalidate the user's intent to close.
+
+    Implementation: install a MockLLMClient whose ``stream`` raises
+    CancelledError on the first iteration. The classifier calls (parse,
+    addressee, scene_close, per-POV summaries) are still served from
+    the canned queue. The post_turn route ultimately re-raises
+    CancelledError after recording the partial — TestClient surfaces
+    that as an exception, so we drive the request inside ``with
+    pytest.raises``. Despite the exception, the scene_closed event
+    must land in the event_log.
+    """
+    from typing import AsyncIterator, Sequence
+
+    _seed_chat_with_guest(tmp_path / "test.db")
+    canned_parse = json.dumps(
+        {"segments": [{"kind": "narration", "text": "we are done here, fade out"}]}
+    )
+    pov_payload = json.dumps(
+        {
+            "summary": "BotA noticed the day winding down.",
+            "knowledge_facts": [],
+            "relationship_summary": "warmer",
+        }
+    )
+    pov_payload_guest = json.dumps(
+        {
+            "summary": "BotB watched the scene close.",
+            "knowledge_facts": [],
+            "relationship_summary": "warmer",
+        }
+    )
+    # Canned queue: parse + addressee + 6 state-updates +
+    # scene_close=True + 2 per-POV summaries. NO interjection slot
+    # because the cancel path short-circuits the interjection branch.
+    canned = [
+        canned_parse,
+        json.dumps(
+            {"addressee_id": "bot_a", "confidence": "medium", "reason": "host"}
+        ),
+        # NOTE: no narrative slot — the stream is hijacked below to
+        # raise CancelledError on first iteration; it never pulls a
+        # canned response.
+        _zero_state(), _zero_state(), _zero_state(),
+        _zero_state(), _zero_state(), _zero_state(),
+        json.dumps({"should_close": True, "reason": "fade out signaled"}),
+        pov_payload,
+        pov_payload_guest,
+    ]
+
+    class _CancelOnStreamMock:
+        """Mock LLM client that serves ``generate`` from a canned queue
+        and raises CancelledError on the FIRST iteration of ``stream``.
+
+        Mirrors :class:`chat.llm.mock.MockLLMClient` for ``generate`` but
+        diverges on ``stream`` to simulate a mid-stream cancel.
+        """
+
+        def __init__(self, canned: list[str]) -> None:
+            self._canned = list(canned)
+
+        async def generate(
+            self, messages: Sequence, *, model: str, **params
+        ) -> str:
+            return self._canned.pop(0)
+
+        async def stream(
+            self, messages: Sequence, *, model: str, **params
+        ) -> AsyncIterator[str]:
+            # Yield a CancelledError on first iteration to simulate the
+            # /turns/cancel route firing mid-stream.
+            raise asyncio.CancelledError
+            yield  # pragma: no cover — keeps this an async generator.
+
+    from chat.web.kickoff import get_llm_client
+
+    mock = _CancelOnStreamMock(canned=list(canned))
+    app.dependency_overrides[get_llm_client] = lambda: mock
+
+    try:
+        # FastAPI/Starlette handles the re-raised CancelledError as an
+        # internal failure — TestClient surfaces it as a 500 response.
+        # We don't assert on the status here; the regression is whether
+        # the scene_closed event still landed in the event_log.
+        try:
+            app_state_setup.post(
+                "/chats/chat_bot_a/turns",
+                data={"prose": "we are done here, fade out"},
+            )
+        except BaseException:
+            # Some Starlette/asyncio versions propagate the
+            # CancelledError out of the test client; that's fine — the
+            # partial-record + scene-close still ran before the raise.
+            pass
+    finally:
+        app.dependency_overrides.clear()
+
+    with open_db(tmp_path / "test.db") as conn:
+        scene_close_count = conn.execute(
+            "SELECT COUNT(*) FROM event_log WHERE kind = 'scene_closed'"
+        ).fetchone()[0]
+        assistant_payload = conn.execute(
+            "SELECT payload_json FROM event_log "
+            "WHERE kind = 'assistant_turn' ORDER BY id"
+        ).fetchall()
+
+    # Scene close lands despite the cancel.
+    assert scene_close_count == 1
+    # The cancelled assistant_turn was still recorded (truncated=True).
+    assert len(assistant_payload) == 1
+    assert json.loads(assistant_payload[0][0])["truncated"] is True
+
+
 def test_interjection_enqueues_significance_job(app_state_setup, tmp_path):
     """T74.2: when an interjection fires, the interjection memory is
     enqueued for significance scoring just like the primary memory.