CoW measurement on CPython 3.14: forked idle child keeps ~71MB shared,
dirties ~1MB private; working child ~26MB. PEP 683 immortal objects keep
code/module pages clean so gc.freeze() is unnecessary (freeze==nofreeze).
prefork.run_fleet: master imports the base floor once, forks one child
per worker (own process/GIL, CoW-shared floor), reaps + restarts with
backoff, graceful SIGTERM->SIGKILL shutdown. Not yet wired to a command
(that lands when 1.2 picks the target worker set).
