exec3/remove-rwtx-threading-merge-main ~34% slower than bal-devnet-3 on SSTORE-bloated benchmark — for visibility

## Summary

Flagging for visibility — the `exec3/remove-rwtx-threading-merge-main` branch is ~34% slower than `bal-devnet-3` on the SSTORE-bloated benchmark despite identical I/O profiles. Likely expected given the in-flight refactoring on that branch, but worth noting before any merge so the regression isn't lost.

## Measurement

Same fixture (issue #20920 SSTORE-bloated `test_sstore_bloated[10GB-fork_Osaka-NO_CACHE-existing_slots_True-write_new_value_True-30M]`, perf-devnet-3 block 24358306), same hardware (`dev-bm-e3-ethmainnet-n4`), same datadir master, no docker / no overlayfs, parallel exec engaged in both runs:

| Branch | mgas/s | wall |
|-------------------------------------------------------------------------|-------:|--------:|
| `bal-devnet-3` (26cd9a07f0) | 2.00 | 14.92s |
| `exec3/remove-rwtx-threading-merge-main` (2a2667a791c8c) | 1.49 | 19.93s |

## I/O is identical, delta is CPU-side

Per-mmap'd-file Rss deltas show the same dominant files in both runs (commitment `.kvi` + `.kv`, storage history `.ef`, latest storage `.kv`), with commitment file sizes essentially identical across branches:

| File | rwtx Run | bal-devnet-3 Run |
|-----------------------------------|---------:|-----------------:|
| `v2.0-commitment.7488-7492.kvi` | +306 MiB | +299 MiB |
| `v2.0-commitment.7492-7494.kvi` | +188 MiB | +182 MiB |
| `v2.0-storage.7488-7492.kv` | +152 MiB | +152 MiB |
| `v2.0-commitment.7494-7495.kvi` | +128 MiB | +118 MiB |
| `v2.0-commitment.7488-7492.kv` | +106 MiB | +106 MiB |

Per-IOP size identical at ~4 KB. chaindata MDBX trivial in both. So the I/O work the trie does is unchanged; the wall-time delta is CPU.

## Run data

- `/home/erigon/perf_results/sstore-bench/run-a-baseline-165007/` (`exec3/remove-rwtx-threading-merge-main`)
- `/home/erigon/perf_results/sstore-bench/run-b-bal-devnet-3-172728/` (`bal-devnet-3`)

## What this is / isn't

Likely **expected** given the parallel-exec refactoring in flight on `exec3/remove-rwtx-threading-merge-main`. Filing for visibility / future-merge attention rather than as an actionable defect. Not blocking.