The link-watch worker now signals a condition variable when it ends,
whether it stored its stamps or unwound, and Session::wait_link_scan
blocks until then. The place tests' idle helper waits on it once per look
in place of a 5 ms sleep under a 20 s wall-clock bound, and the engine's
link-watch and link-path tests wait on it in place of spinning under a
10 s bound. The link-watch tests date a rewritten file 2 s after the one
it replaces instead of sleeping 30 ms before writing it.