feat: comprehensive performance optimizations with 2.36x improvement for Unpin futures

[czy-29]

Summary

This PR implements comprehensive performance optimizations across multiple modules in the est library, with the most significant improvement being a 2.36x performance boost for Unpin futures.

Key Optimizations

🚀 Future Module - Major Performance Improvement

• New: WithCancelSignalUnpin structure for zero-cost abstraction over Unpin futures
• New: with_cancel_signal_unpin() method avoiding heap allocation
• Benchmark: 2.36x faster than boxed version (60ns vs 142ns)
• Impact: Significant performance improvement for async code using Unpin futures

🔧 Collections Module - Bug Fix + Optimization

• Fixed: HashMap/BTreeMap replace_key error handling order to maintain API contract
• Added: #[inline] hints for better compiler optimization
• Impact: Maintains correctness while improving hot path performance

⚡ Process Module - Reduced Overhead

• Optimized: Clone implementation using pattern matching to avoid redundant conversions
• Improved: Environment variable processing with batch operations
• Impact: Reduced CPU overhead for process command operations

🎯 Sync Module - Hot Path Optimization

• Added: #[inline] hint to trigger() method
• Impact: Better performance for critical synchronization operations

Benchmark Results

with_cancel_signal_unpin: 60.29 ns
with_cancel_signal_boxed: 142.33 ns
Improvement: 2.36x faster

Testing & Quality Assurance

• ✅ All 28 unit tests pass
• ✅ All 21 documentation tests pass (1 ignored)
• ✅ Clippy analysis clean (no warnings)
• ✅ Full backward compatibility maintained
• ✅ Comprehensive benchmarks added

Documentation

• Added comprehensive performance optimization report (PERFORMANCE_OPTIMIZATIONS.md)
• All new APIs properly documented with examples
• Benchmark suite added for future performance tracking

Compatibility

• API: Fully backward compatible
• Rust Version: Compatible with Rust 1.85.0+
• Dependencies: No new required dependencies
• Features: All feature flags work as expected

Files Changed

• src/collections.rs: Fixed error handling order + inline optimizations
• src/future.rs: Added WithCancelSignalUnpin + inline optimizations
• src/process.rs: Optimized Clone implementation
• src/sync/once.rs: Added inline hint to trigger method
• Cargo.toml: Added criterion for benchmarking
• benches/performance.rs: Comprehensive benchmark suite
• PERFORMANCE_OPTIMIZATIONS.md: Detailed optimization report

These optimizations provide measurable performance improvements while maintaining the library's high code quality standards and full backward compatibility.