Canonical ex4pm workflow facade over Ash.Reactor and Reactor.

use Ex4pm.Reactor delegates directly to Ash.Reactor; ex4pm does not own a competing DSL, planner, scheduler, retry loop, or saga executor. Dynamic POWL models are lowered with Ex4pm.Runtime.compile/1 into the same Reactor runtime.

Ash-aware workflows therefore retain the native Ash Reactor action surface, while ex4pm contributes process semantics, BRCE authority, receipts, replay, mining, and conformance around that single execution substrate.