Single-process, in-memory implementation of the three Authority contracts (implementation.md §3.2). It exists for three reasons: executable reference semantics for the contracts, unit tests without Postgres, and single-node development. Not for production multi-node use — it is itself a SPOF with none of Postgres's durability.

Lease and leadership expiry are judged on Fief.Seam read in this process, so under simulation the arbiter's clock is virtual and steppable — and may be skewed against per-node virtual clocks (design §6.4). Test seams (undocumented, test-substrate only):

• :sim_clock start opt — binds a simulation clock (a Fief.Sim.ManualClock or a Fief.Sim.Scheduler.clock/3 handle) as the arbiter's clock.
• set_unreachable/3 — makes node-scoped operations return {:error, :unreachable} for one node (or :all), simulating an arbiter that cannot be reached without expiring anything.
• try_lead/3 / leader/1 — leadership storage primitives with monotonic term issuance; the notify-loop Fief.Leadership adapter (M3) campaigns through these.