prisma-risk / tsoracle / 26374735542

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refactor(driver-paxos): extract ApplyEngine + ApplyTask from StandaloneHost (#327)

StandaloneHost carried the apply state, snapshot policy, the spawned apply
task's lifecycle, and the synchronous stepping path inline: an 11-field
struct whose start() closure duplicated apply_once()'s drain+snapshot four
lines, and whose apply_handle / apply_shutdown were two Mutex<Option<..>>
kept in lockstep only by convention.

Extract a pub(crate) apply module:

- ApplyEngine bundles apply_state + snapshot policy and owns the single
  drain+snapshot step (apply_step) shared by the sync stepping path and the
  async apply task, a drain-only recover() for construction (recovery stays
  snapshot-free), and the accessors the barrier-linearized reads use.
- ApplyTask owns the join handle + per-spawn shutdown Notify; stop(self)
  consumes it, making double-stop unrepresentable.

StandaloneHost drops to 8 fields: apply_state + policy collapse into one
engine, and apply_handle + apply_shutdown collapse into one
Option<ApplyTask> (no Mutex, since start/stop take &mut self), so a
half-installed apply task is structurally impossible. The public method
surface and the PaxosHighWaterHost impl are unchanged; every method is now a
thin delegator.

Behavior-preserving internal refactor: no public API change. The yieldpoint
label and the snapshot-free recovery path are preserved; all existing tests
pass unchanged.

95 of 96 new or added lines in 2 files covered. (98.96%)

11411 of 12027 relevant lines covered (94.88%)

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