dune serve stays alive after a graceful shutdown when anything holds the event loop #47

After SIGTERM, dune serve closes the listener, drains, prints shutdown complete and then just returns. Any ref'd timer keeps the process alive indefinitely at 0% CPU with no listener; a second SIGTERM is needed to end it. About 45 such processes accumulated on one developer machine.

The trigger found was @dune/plugin-admin's collab eviction setInterval (duneorg/plugin-admin#8), but core shouldn't depend on every plugin unref'ing its timers.

Reproduced: one SIGTERM leaves serve alive for 24 s+; process.getActiveResourcesInfo() after shutdown showed exactly one Timeout.

Fix in #38: exit explicitly once the drain is done.