* test(#4020): the runner must bound and sweep a run-scoped temp root
* fix(#4020): bound the run's temp footprint with a swept run-scoped root
* test(#4020): isolate the env-mutating rows in child processes
* fix(#4020): gate root removal on ownership so nested runners spare the outer root
* test(#4020): pass the probe file via --files, the runner's explicit-file flag
* test(#4020): resolve the probe by basename, as --files matching requires
* test(#4020): assert root survival, not content survival, in the nested-row
* chore(#4020): changeset for the run-scoped temp root
* chore(#4020): backfill changeset pr number
* fix(#4020): the sweep spares ancestors of the runner's own selected files
* fix(#4020): TMPDIR precedence — an operator redirect beats inherited TEMP/TMP
* fix(#4020): only the root's owner sweeps — a nested runner spares live sibling fixtures
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Co-authored-by: sim <sim@local>
