Unified Clock trait for the Harn runtime.
Production code that needs the current time, monotonic measurement, or a
cancellable sleep takes an Arc<dyn Clock> and reads through it. Real
deployments wire [RealClock]; tests substitute [PausedClock] to drive
virtual time deterministically; recording layers wrap any inner clock with
[RecordedClock] to capture every observation for replay.
See docs/src/dev/testing.md for usage patterns.