feat: regenerate registers stream task in _in_flight_tasks (T83.1)

Both the primary and the interjection sub-stream in
``regenerate_assistant_turn`` are now wrapped in ``asyncio.create_task``
and registered in the chat-keyed ``_in_flight_tasks`` registry that the
``/turns/cancel`` route reads. Without this, hitting Stop during a
mid-regenerate stream was a no-op.

Mirrors the meanwhile registration pattern in chat/web/meanwhile.py
(snapshot-tested by tests/test_meanwhile_turn_flow.py).

Test added: test_regenerate_registers_task_in_in_flight_tasks captures
``"chat_bot_a" in _in_flight_tasks`` at the first stream yield via a
custom MockLLMClient subclass and asserts post-flight cleanup.

chat/services/regenerate.py

@@ -68,6 +68,7 @@ Phase 2.5 changes:
 
 from __future__ import annotations
 
+import asyncio
 import json
 from sqlite3 import Connection
 
@@ -250,19 +251,37 @@ async def regenerate_assistant_turn(
         guest_id=guest_bot_id,
     )
 
-    # 5. Stream the new narrative.
+    # 5. Stream the new narrative. T83.1: register the streaming Task in
+    # the chat-keyed in-flight registry so POST /chats/<id>/turns/cancel
+    # can call ``.cancel()`` on a mid-regenerate stream. We import the
+    # underscore name from turns.py deliberately — same single-process
+    # registry the cancel route reads, mirrors the meanwhile registration
+    # pattern in chat/web/meanwhile.py.
+    from chat.web.turns import _in_flight_tasks  # noqa: PLC0415
+
     accumulated: list[str] = []
-    async for chunk in client.stream(
-        messages,
-        model=settings.narrative_model,
-        max_tokens=settings.narrative_max_tokens,
-        temperature=settings.narrative_temperature,
-    ):
-        accumulated.append(chunk)
-        await publish(
-            chat_id,
-            {"event": "token", "text": chunk, "speaker_id": speaker_bot_id},
-        )
+
+    async def _stream_primary() -> None:
+        async for chunk in client.stream(
+            messages,
+            model=settings.narrative_model,
+            max_tokens=settings.narrative_max_tokens,
+            temperature=settings.narrative_temperature,
+        ):
+            accumulated.append(chunk)
+            await publish(
+                chat_id,
+                {"event": "token", "text": chunk, "speaker_id": speaker_bot_id},
+            )
+
+    stream_task = asyncio.create_task(_stream_primary())
+    _in_flight_tasks[chat_id] = stream_task
+    try:
+        await stream_task
+    finally:
+        # Always unregister so a subsequent turn / regenerate can register
+        # a fresh task. Mirrors the cleanup in turns.py::post_turn.
+        _in_flight_tasks.pop(chat_id, None)
     new_text = "".join(accumulated)
 
     # 6. Append the new assistant_turn event. ``user_turn_id`` points at
@@ -497,21 +516,32 @@ async def regenerate_assistant_turn(
             )
 
             interject_accumulated: list[str] = []
-            async for chunk in client.stream(
-                interject_messages,
-                model=settings.narrative_model,
-                max_tokens=settings.narrative_max_tokens,
-                temperature=settings.narrative_temperature,
-            ):
-                interject_accumulated.append(chunk)
-                await publish(
-                    chat_id,
-                    {
-                        "event": "token",
-                        "text": chunk,
-                        "speaker_id": silent_witness_id,
-                    },
-                )
+
+            async def _stream_interjection() -> None:
+                async for chunk in client.stream(
+                    interject_messages,
+                    model=settings.narrative_model,
+                    max_tokens=settings.narrative_max_tokens,
+                    temperature=settings.narrative_temperature,
+                ):
+                    interject_accumulated.append(chunk)
+                    await publish(
+                        chat_id,
+                        {
+                            "event": "token",
+                            "text": chunk,
+                            "speaker_id": silent_witness_id,
+                        },
+                    )
+
+            # T83.1: register the interjection sub-stream in the same
+            # in-flight registry so /turns/cancel collapses it too.
+            interject_task = asyncio.create_task(_stream_interjection())
+            _in_flight_tasks[chat_id] = interject_task
+            try:
+                await interject_task
+            finally:
+                _in_flight_tasks.pop(chat_id, None)
             interject_text = "".join(interject_accumulated)
 
             new_interjection_event_id = append_event(