Design and benchmark Pebble KVS for state store

Pebble KVS is a performant key value store. It's most likely a better fit for the state db.

- We need two query modes: latest version (no block constraint) and historic (latest version where BlockNum ≤ X)
- Read-only snapshot isolation via db.NewSnapshot(): the state may not change during one endorsement. Snapshot = point-in-time, immune to concurrent writes
- Two snapshot types: current (latest) and virtual (pinned to a BlockNum)
- Version semantics are backend-dependent (monotonic for fabric-x or equals BlockNum for fabric). We must learn the version when we read a key.
- Must persist durably and safely (a crash while processing a block must not lead to inconsistent state) - either using transactions or idempotency.

Start by researching Pebble KVS and mapping its query modes, snapshot isolation, version semantics, and durability options to the requirements in this issue. Done means a benchmarked state-store design that supports current and BlockNum-pinned reads and remains consistent after a crash; the issue does not name files or tests to run.

Written by the indexing model from the issue text.