Added the POSIX unsetenv() function and its internal support. Implemented EnvironmentManager::unset() to remove a variable by name, free the string if allocated, and compact the array. Updated EnvironmentManager to synchronize the public global environ pointer when transitioning to managed storage. Registered for x86_64, aarch64, and riscv. Integration tests cover basic operations and edge cases. Assisted-by: Automated tooling, human reviewed.