Add JMH benchmark subproject for HashSet vs BitSet performance comparison

[Copilot]

Adds comprehensive JMH benchmarks to quantify performance characteristics of the custom BitSet implementation against standard HashSet across common use cases.

Changes

• New benchmarks Maven module with JMH 1.37, configured to produce standalone executable JAR via Maven Shade plugin
• Six benchmark classes covering operation types:
  • SetAddBenchmark - insertion performance
  • SetContainsBenchmark - lookup performance
  • SetIterationBenchmark - traversal performance
  • SetRemoveBenchmark - deletion performance
  • SetMemoryBenchmark - allocation patterns with GC profiling
  • SetMixedOperationsBenchmark - realistic workload simulation
• Parameterized tests across data sizes (100, 1K, 10K, 100K elements) and fill ratios (50%, 90%)
• Documentation in benchmarks/README.md covering execution, profilers, and result interpretation

Usage

mvn package -pl benchmarks -am
java -jar benchmarks/target/benchmarks.jar

# Specific benchmark with custom parameters
java -jar benchmarks/target/benchmarks.jar SetAddBenchmark -p size=10000 -p fillRatio=0.9

# With GC profiler
java -jar benchmarks/target/benchmarks.jar -prof gc

Notes

• Java 21 required for benchmark module (overrides parent POM's Java 25 target)
• Initial test run shows ~7.6x speedup for BitSet add operations on small datasets
• All benchmarks use fixed random seed (42) for reproducibility

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