A draining variant of PHOLD (spec §14.1). Each processed event schedules one
follow-up to a random LP at at + delay — but only if that time is still below
the horizon. Tokens therefore fall off the end of the horizon over time and the
population drains to zero, driving the system to genuine quiescence.
Where the constant-population TimeWarp.Examples.PHOLD relies on the run
horizon to stop, this model reaches natural termination — exercising the
Coordinator's quiescence detection (Phase-0 GVT) rather than the until cutoff.
Purity is honoured exactly as in PHOLD: all randomness is drawn from a
per-LP TimeWarp.Rand carried in model_state.