feat(shutdown): add phased ShutdownBus with per-phase drain budgets

ShutdownBus (bus.rs) replaces the flat ShutdownWatch signal with a
structured drain sequence. Subsystems register named drain tasks per
ShutdownPhase, and the bus enforces a 500ms budget per phase before
aborting laggards and logging an offender report.

The flat ShutdownWatch is preserved as the underlying signal so all
existing watch::Receiver<bool> subscribers continue to work without
change.

Author: farhan-syah