[MM] Refcounted pages, anonymous mmap/brk, and copy-on-write fork
This merge request enhances the memory management subsystem with modern virtual memory features:
- Refactored physical frame allocator with per-page reference counting
- Added anonymous mmap system call support
- Improved brk to support dynamic user heap growth
- Implemented copy-on-write fork by removing write permissions and handling data duplication in the page fault handler
- Added user-level tests for mmap and COW behavior
These changes enable efficient memory sharing and lazy copying between processes.