Add comprehensive stress tests for BTree and BPlusTree implementations
- Introduced a new stress test suite in BTree.Test/BTree/StressTest.cs to validate the integrity and performance of BTree and BPlusTree under various scenarios.
- Implemented tests for large dataset insertion and removal, random insert/remove operations, alternating insert/remove patterns, and sequential operations.
- Enhanced BPlusTree and BTree classes to support efficient leaf node traversal and iteration using callback interfaces for high-performance scenarios.
- Updated project files to support multiple target frameworks (netstandard2.0, net6.0, net8.0, net10.0) and incremented version to 4.0.0.
- Added new interfaces for allocation-free iteration over BTree items to minimize delegate overhead.
- Improved the Option struct for better performance in hot paths.
- Updated README.md to reflect changes and provide clearer usage instructions.