SessionService reads the current session from XDG_STATE_HOME. On a build
host whose user had an auto-created session ("auto: /data") with watch
enabled, every test that booted a ServiceManager inherited it, the session
watcher walked /data during init, and six daemon tests timed out. With the
state dir isolated the same six pass.
The shared test main now points XDG_STATE_HOME at a per-process temp dir
when the caller did not set one, and removes it at exit. A small test checks
the process sees no host session.
Signed-off-by: trvon <me@trevon.dev>
