fix(examples): use real keccak256 for StorageWritten event topic

[cuiweixie]

Summary

• Replace the placeholder topic0 bytes in the custom precompile journal example with keccak256(b"StorageWritten(address,uint256)").
• Add a regression assertion that the emitted log's first topic matches that signature hash.

Test plan

• cargo nextest run -p example-custom-precompile-journal --lib
• cargo clippy --workspace --all-targets --all-features

[codspeed-hq]

Merging this PR will degrade performance by 3.12%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

❌ 1 regressed benchmark
✅ 217 untouched benchmarks
⏩ 9 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	ecrecover precompile	392.1 µs	404.8 µs	-3.12%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing cuiweixie:fix/example-storage-written-event-topic (8611359) with main (8ca4052)}

Footnotes

1. 9 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩