Moving TMPDIR into the private runtime root left pytest's default
<TMPDIR>/pytest-of-<user>/pytest-<n> beneath it. With xdist's popen-gw<n>
the real-tmux witness bound a 110-byte socket path, over Linux's 107-byte
sun_path limit, so it failed under every worker count while passing
serially.
The controller now roots basetemp at the private root's pytest directory;
xdist hands workers popen-gw<n> beneath it. An explicit --basetemp wins.
A tmux-independent witness binds a socket at the same path budget.
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