WasmVM: WASI pthreads stub limitations (mutex, condvar, guardsize)

[NathanFlurry]

Several os-test pthread conformance tests fail because the WASI pthreads stub does not support:

• pthread_attr_getguardsize (real thread stack guard pages)
• pthread_condattr_getclock / pthread_condattr_setclock (condition variable clock attributes)
• pthread_mutex_timedlock (timed mutex)
• pthread_mutex_trylock with ERRORCHECK (mutex contention detection)
• pthread_mutexattr_setrobust (robust mutex owner-died detection)
• pthread_mutexattr_settype ERRORCHECK/RECURSIVE (mutex type enforcement)

Category: wasm-limitation — these require real threading support not available in single-threaded WASM.

7 tests affected.