Function lock_library 

pub fn lock_library()

Lock the library (global mutex).

UPSTREAM-PARITY

void xmlLockLibrary(void);

In upstream libxml2, this locks a global mutex. In Rust, this is a no-op because Rust’s type system prevents data races. However, for FFI safety with C callers that may manipulate shared state, a real mutex would be needed. This will be enhanced in Phase 2+.