feat(benchmarking): add mixed workload benchmark (TestMixedTraffic)

[chatton]

Summary

Add a benchmark test that runs multiple spamoor scenarios simultaneously against the same chain to simulate realistic L2 traffic patterns. Currently each benchmark test isolates a single workload type, which measures that workload ceiling but does not represent production traffic mixes.

Motivation

The DeFi simulation benchmark (TestDeFiSimulation) is bottlenecked by spamoor uniswap-swaps scenario - each swap requires a synchronous eth_call to GetAmountsIn/GetAmountsOut before sending, limiting injection to ~150-300 TPS regardless of config. A mixed workload avoids this single-scenario bottleneck while producing more realistic throughput numbers.

Proposed workload mix

Spammers	Scenario	Gas/tx	Role
2	eoatx	~21K	simple transfers, high TPS filler
2	erc20tx	~65K	storage-heavy contract calls
1	uniswap-swaps	~200K	DeFi call chains
1	gasburnertx	configurable	compute pressure

This mix exercises simple transfers, storage reads/writes, deep call chains, and compute simultaneously - closer to what a production L2 mempool looks like.

Acceptance criteria

• New TestMixedTraffic in test/e2e/benchmark/
• Uses benchConfig env vars for system config (block time, gas limit, etc.)
• All spammer types run concurrently against the same chain
• Measurement window excludes warmup from all scenarios
• Results include per-scenario and aggregate metrics

Related

• Epic: [EPIC] Benchmarks #2288
• DeFi benchmark: cian/add-defi-simulation-benchmark
• Spamoor bottleneck analysis: see PR for TestDeFiSimulation