The driver's clock. Every internal that measures elapsed time — pool
idle/lifetime/acquisition, connection deadlines, routing-table ttl,
managed-tx retry budget — reads it instead of calling Process/Time
directly, and gets it injected (DriverFactory#create_clock, threaded
through as its own clock: parameter). This default reads the real monotonic
and wall clocks; testkit-backend supplies its own via the same seam
(see Internal::ClockAdapter). The internals are agnostic to which clock
they hold — there is no notion of "real" vs "mock" below this line.