runtime_python() released .prepare.lock on return and the worker/CLI child
only leased its generation once its own code ran. A publish plus `pm gc` in
that window could remove the generation the child was starting from.
The resolving process now leases the generation it returns while still
holding .prepare.lock (which the collector also takes), once per generation
for the life of the process, so the child is covered until it holds its own.
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