Moves the temp-home guard (`temp_home_in_service_definition`,
`refuse_temp_home_service_write`) and the systemd install's ownership
resolution (`unit_belongs_to_caller`) out of the gateway.py facade into
the owner sibling, so the facade shrinks instead of growing. Narrows the
plist read's catch-all to the errors plistlib raises. Temp-home tests
move with their code into test_gateway_service_owner.py.
1e0c7730d7