Add kotlinx-benchmark for performance testing

[thebehera]

Summary

This PR adds kotlinx-benchmark (JMH wrapper) for proper microbenchmarking across all platforms.

Changes

• Added kotlinx-benchmark plugin and runtime dependency
• Configured benchmark targets for JVM, JS, macOS, Linux, and WASM platforms
• Added benchmark classes in commonTest:
  • BufferBenchmark - Basic allocation benchmarks
  • PlatformBufferBenchmark - Read/write operations and slicing
  • FragmentedReadBufferBenchmark - Fragmented buffer performance

Usage

# Run JVM benchmarks
./gradlew jvmTestBenchmark

# Run macOS benchmarks
./gradlew macosArm64TestBenchmark

# Run JS benchmarks
./gradlew jsTestBenchmark

Test plan

• JVM benchmarks run successfully
• Verify other platform benchmarks compile and run