Cherry-pick of helix4u's #88836 (75742db5a42) resolved onto current main.
The POSIX runtime cutover parks the live venv with a directory rename. On
Windows that can never succeed from `hermes update`: the updater executes
from venv\Scripts\python.exe and Windows keeps that image mapped, so the
rename fails with WinError 5 on every attempt (#93032). Instead, atomically
replace the live venv's pyvenv.cfg with the candidate's: Scripts\python.exe
is a launcher that reads `home` on every start, so every fresh process runs
the new generation while nothing touches the locked directory. A failed
post-cutover smoke restores the original config; a failed rollback keeps
the generation the live config now references.
5a49d1de3d