Cover room termination: a deleted room or a removed member must not be nudged by a late completion #318

From the #198 acceptance audit: the 验收与证据 box "终止/离开的房间不得继续触发工作" had zero test coverage — the closest analog was "deleting the responsible Bot fails its open tasks".

The behavior exists in the host (verified by reading and by mutation):

• Layer 1: turns.js ignores session events from a session that is no longer a bot session (dropGroupSession unlinks it when the room is deleted or the member removed).
• Layer 2: lateRoomReply/lateRoomFailure re-check the room and membership before posting.
• deleteRoom also clears the room inbox and archives the room's own session with stopActivity.

Nothing pins it. A regression in either layer would let a mid-work member's late completion restart a round in a room that no longer exists or no longer contains it.

Acceptance:

• Same-room fixture tests: delete the room mid-work (member working, round moved on), then deliver the member's late completion — assert no followup reaches the room, the relay for the deleted room refuses (Not a group chat), and nothing dispatches.
• Remove the member mid-work, then deliver its late completion — assert no post lands in the room log, no room nudge, lateRounds untouched.
• Mutation-checked: removing both containment layers makes the new tests fail (single-layer removal still passes, i.e. the defense in depth is real).