Replace the {Account, Storage, Code} time.Duration scalars threaded through
ProcessResultWithMetrics, txExecResult, processBlockPreTx and resultHandler
with a single ReadDurations struct + Add merge primitive. Same shape as
StateCounts. Adds (*StateDB).SnapshotReads() helper at the boundary.
The struct is 80 bytes (10 ints) — value semantics matches the type's
"snapshot, safe to pass by value" thesis stated in its doc comment, and
removes three unnecessary &-takings at call sites. No behavior change.
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).
