A door's own launches retire the door under load and cut every in-flight stream #441

Sessions intermittently print "Streaming response ended before any complete data was received. Retrying without streaming." The door logs show it was replaced 29 times in two hours under heavy load, against about one an hour normally, and each replacement drops every in-flight stream across all sessions at once.

The replacement is the door's own doing. A launch run inside the door (the usage refresh, the window opener, a job) asks ensureFrontDoor whether the door it plans to use is alive. Asked about itself, that ran a synchronous ps with no timeout and a synchronous probe, both holding the door's own event loop. Under load they take seconds, the probe cannot be answered, it reads as a dead door, and the same launch retires it when no other session is registered.

This is a different path from #420, #424 and #370: those fixed retiring a busy door for a launcher outside it and probing the door a launch is planned inside, but a door's own launch still asked ps and probed its own loop. #439 makes the supervisor declare itself the front door at start so it asks nothing about itself, and #426 and #427 cover the structural guard and the watchdog.

Verification that closes this: after the door is replaced with a build carrying #439, the door replacement count under comparable load stays at its normal rate and the error stops appearing in new sessions.