perf: parallelize HashedPostStateSorted::from_reverts hashing/sorting

[Andrurachi]

Summary

Introduces a parallelized path for HashedPostStateSorted::from_reverts using rayon, gated behind a new parallel-from-reverts feature flag.

This optimization targets CPU bottlenecks caused by deep reorgs or blocks with skewed storage distributions. It implements an Account Count Threshold (2,500) to ensure no regressions on small or standard blocks.

Motivation

Closes #20049

Processing large state reverts involves two distinct phases:

1. DB Cursor Walk (I/O Bound): Sequential reading of account/storage changesets.
2. Sorting & Hashing (CPU Bound): Organizing the raw data into a sorted state.

While the DB walk must remain sequential, the sorting phase becomes a bottleneck when thousands of accounts (or accounts with massive storage slots) need processing. This PR parallelizes the CPU-bound sorting phase, reducing wall-time for heavy blocks.

Implementation Details

1. Feature Flag: Added parallel-from-reverts (opt-in).
2. Strategy: Parallelize the outer loop (iterating over accounts) rather than the inner loop (sorting slots). Benchmarks proved that inner-loop parallelization (par_sort_unstable) introduced too much overhead for typical slot counts (< 1,000).
3. Threshold: A threshold of 2,500 Accounts was established via micro-benchmarking.
   • < 2,500: Always Sequential (avoids Rayon overhead).
   • >= 2,500: Parallel (distributes load, handles skewed accounts).
4. Logic Preservation: Preserves the existing eager hashing logic (hashing keys during collection) to ensure cache locality and consistency with the sequential path.

Benchmarks

1. Micro-Benchmarks (sorting_par_exp)

Measured purely the sorting/hashing overhead (excluding DB reads).

Scenario	Accounts	Slots/Acc	Seq Time	Par Time	Speedup
Acc_Low	100	4	8 µs	124 µs	-15x
Acc_Med	1,000	4	73 µs	122 µs	-0.6x
Acc_High	10,000	4	790 µs	612 µs	1.3x
Skewed_10k	10k	Mixed	64 ms	14 ms	4.6x

Skewed Distribution: 95% accounts have 4 slots, 5% have 2,000 slots.*

2. Integration Benchmarks (integration_bench)

Measured full lifecycle: DB Read -> Hashing -> Sorting -> Allocation.

Scenario	Description	Seq Time	Par Time	Delta
Small	1k Acc (Below Threshold)	61.2 ms	61.0 ms	0% (No Regression)
Large_Uniform	10k Acc (20 slots)	270 ms	267 ms	~1.2%
Large_Skewed	10k Acc (Whale Dist.)	1,439 ms	1,363 ms	~5.3%

Since the total runtime is dominated by DB I/O, this actually represents a solid optimization of the available CPU-bound work. The threshold ensures no regression for small blocks.

Checklist

• Added rayon dependency (optional).
• Implemented parallel-from-reverts feature flag.
• Added benchmarks covering Uniform, Skewed, and Threshold scenarios.
• Verified no regressions for inputs below the 2,500 account threshold.