[FEATURE] Add ERC20 throughput benchmark test

[chatton]

Problem to solve

No dedicated benchmark test for ERC-20 token transfer throughput.

Proposed solution or feature

Add TestERC20Throughput to measure ERC-20 token transfer throughput. Uses spamoor to generate load and reports MGas/s, TPS, and per-span latency breakdown.

Alternatives considered

N/A

Additional context

Part of #2288.