`_ensure_uv_path` accepted `$HERMES_HOME/bin/uv[.exe]` on is_file()+X_OK
alone, so a dead relocated launcher a pre-fix installer had salvaged there
was handed to `hermes update` and doctor/repair forever; the only self-heal
lived in install.ps1. Probe it with `uv --version` (exit code, OSError =
dead) and fall through to the existing `_install_uv` when it fails.
Invariant test: an executable exit-1 stand-in is replaced (red before:
`_install_uv` called 0 times; green after). The Windows plain-str test
answers the probe for its shell-script `uv.exe` stand-in.
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