prisma-risk / tsoracle / 26319868546

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feat(driver-paxos): StandaloneHost and PaxosDriver public façade (#182)

* feat(driver-paxos): StandaloneHost with apply pipeline and builder

StandaloneHost owns an OmniPaxos handle, the toolkit's PaxosRunner, a
spawned apply task, and the in-memory ApplyState. It implements
PaxosHighWaterHost so the driver can drive it directly.

The apply task awaits PaxosRunner::apply_notify, drains the decided
suffix into the AtomicU64 high-water, and triggers compaction via the
configured SnapshotPolicy. Blocking-read methods (current_high_water,
submit_advance) snapshot decided_idx, append a Barrier or Advance, and
loop on ApplyState::apply_notifier until their wait condition holds.

StandaloneHostBuilder provides an ergonomic construction surface with
required-field validation (omnipaxos handle, my_node_id) and sensible
defaults for tick_interval and snapshot policy.

* feat(driver-paxos): PaxosDriver wrapping any host, with fence + epoch re-derivation

PaxosDriver implements tsoracle_consensus::ConsensusDriver against any
PaxosHighWaterHost. The wrapper layer is thin and serves two purposes
that the toolkit runner cannot satisfy on its own:

1. Leader-event epoch re-derivation. The toolkit emits Leader events
   with a process-local counter-based placeholder epoch; the driver
   maps the stream to derive epoch via encode_epoch(omnipaxos.get_promise())
   so the value matches what persist_high_water's fence check expects.

2. Fence pre-flight. persist_high_water compares the supplied epoch
   against the current ballot-derived epoch and returns
   ConsensusError::Fenced on mismatch, avoiding a wasted log slot when
   a stale leader attempts to advance. The check + submit are not
   atomic across the OmniPaxos handle, but a stale ballot's append
   would be rejected downstream anyway.

leadership_events() is take-once: subsequent calls return an empty
stream. load_high_water and persist_high_water delegate to the host
after the fence check.

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