The runner's per-run temp roots live under a fixed `/var/tmp/hermes-pytest`, chosen for
real reasons (disk-backed, not hidden, short enough for AF_UNIX sun_path). But a fixed
literal in a world-writable sticky dir belongs to whoever creates it first: a root-owned
root — a container or system-service run — makes every later `makedirs`/`mkdtemp` there
fail with EPERM for every other user on the host, with no way back that does not need
root. That is exactly what happened on luna: /var/tmp/hermes-pytest is root:root 755, so
every local suite run by the login user died at `runner crashed: PermissionError(13)`
before collecting a single test.
Key the name by uid (with the same non-/var/tmp fallback), so no run can be blocked by
another user's leftovers. Invariant test: two uids never share a scratch root, and the
root is created under the expected parent.
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