Adds the StateCounts type that the BAL slow-block work depends on:
- core/state/state_counts.go: 10-field plain-int snapshot type with
Add merge primitive; isolates the live atomic mutation surface from
the value-typed aggregation pipeline.
- core/state/statedb.go: SnapshotCounts() method that converts the
StateDB's atomic counters to a plain StateCounts at the boundary.
- core/blockchain_stats.go: ExecuteStats embeds state.StateCounts;
adds ExecWall/PostProcess/Prefetch BAL extension fields, the
slowBlockBAL JSON struct + BAL field on slowBlockLog, and extracts
buildSlowBlockLog as a pure helper for direct testing.
Without this commit the bal-devnet-3 branch as committed in subsequent
commits would not build for a fresh clone (state.StateCounts undefined).
