process pool: parameterize over the inner Reconcile impl instead of hardcoding OptativeSet

Came up while planning the optative-process-pool extraction.

ProcessPool currently wraps `OptativeSet` directly. The pool only ever calls `inner.reconcile()`, `inner.get()`, `inner.iter()`, and the Drop cleanup. All of those are on the `Reconcile` trait or close to it. Making the pool generic over `R: Reconcile` (with `OptativeSet` as the default) would let users swap in:

- a persistent store backend (state survives restarts)
- a sorted-iteration backend (BTreeMap)
- a mock store for tests

No trait redesign required. The Drop impl works unchanged because it just calls `inner.reconcile(vec![], ...)`.

Drafted by Claude (AI assistant) during a design review.