perf(state): avoid linear validator lookups in MedianTime - #6051

Summary

MedianTime performs a linear validator lookup for each commit signature. Use direct index access when the address matches, with address lookup as a fallback to preserve behavior for out-of-order signatures.

Performance

Apple M4, 300ms per benchmark, median of 5 runs:

These results measure MedianTime with ordered signatures and identical timestamps, not overall node throughput. Allocation counts are unchanged.

Testing

• Added a regression test verifying that out-of-order signatures with unequal voting powers preserve the weighted median.
• Added benchmarks for different validator set sizes.
• Passed go test ./state -count=1.

PR checklist

• Tests written/updated
• Changelog entry added in CHANGELOG.md
• Updated relevant documentation (docs/ or spec/) and code comments