Add thread safety

[el-dockerr]

This pull request introduces comprehensive thread safety to the GhostMem memory manager, updates documentation and versioning to reflect this milestone, and adds a suite of multi-threaded tests to ensure correctness under concurrent usage. The changes include internal synchronization with mutexes, new threading tests, and updated documentation to advertise thread safety.

Thread safety and core implementation:

• Added a std::recursive_mutex (mutex_) to GhostMemoryManager and protected all shared data structures and public methods with this mutex, ensuring thread-safe concurrent access and page fault handling. [1] [2] [3] [4] [5] [6] [7] [8]

Testing and build system:

• Added tests/test_threading.cpp, a comprehensive set of multi-threaded tests covering concurrent allocations, reads/writes, page eviction, compression cycles, and random data manipulation to validate thread safety.
• Updated CMakeLists.txt to include the new threading test and link against pthreads on non-Windows platforms.

Documentation and versioning:

• Updated README.md to highlight thread safety, mark the feature as complete, and remove outdated claims about single-threaded operation. [1] [2] [3]
• Bumped the project version to 0.9.0 in both README.md and Version.h, and updated version tests accordingly. [1] [2] [3]