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feat(driver-openraft): persist HighWaterStateMachine snapshots via pluggable SnapshotStore (#36)

* feat(driver-openraft): persist HighWaterStateMachine snapshots via pluggable SnapshotStore

Adds a `SnapshotStore` trait that `HighWaterStateMachine` writes through
to from `build_snapshot` and `install_snapshot`, and rehydrates from on
construction via the new `with_store` constructor. Ships an
`InMemorySnapshotStore` (default, preserving existing `new()` behavior)
and a `RocksdbSnapshotStore` behind a `rocksdb-snapshot-store` feature
(default-on) keyed in a caller-managed CF with `set_sync(true)` writes.

This is what lets openraft re-enable the default
`SnapshotPolicy::LogsSinceLast(N)` against this state machine without
losing data when the log prefix is purged. The `openraft-standalone`
example switches to the default policy and shares one `Arc<DB>` between
log and snapshot. The `openraft-piggyback` example keeps
`SnapshotPolicy::Never` because its custom `HostStateMachine` is still
in-memory; comments and READMEs updated to point at that constraint
rather than at a "driver-crate follow-up."

Adds `tests/snapshot_restart.rs` exercising the snapshot -> purge ->
restart -> replay sequence: bumps a single-node cluster with
`LogsSinceLast(4)` + `max_in_snapshot_log_to_keep = 0` until openraft
fires a snapshot and purges the log, then reopens and asserts the SM
recovers the last value -- impossible without the persisted snapshot.

Closes #25

* test(driver-openraft): cover Debug, Default, decode-error, and CF-drop paths

Closes the 0.06% coverage delta flagged in the prior commit by hitting
the production-code branches the existing tests left untouched:

- `RocksdbSnapshotStore` `Debug` impl, exercised via `format!("{:?}", ...)`.
- `cf_handle` error path, triggered by dropping the CF after `open`
  succeeded — surfaces as an `io::Error` rather than a panic.
- `HighWaterStateMachine::default()`, the canonical no-arg entry point.
- `begin_receiving_snapsh... (continued)

291 of 293 new or added lines in 2 files covered. (99.32%)

4894 of 5135 relevant lines covered (95.31%)

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