PlatformMutex.native uses pthread_mutex_init, which nativeMain's shared metadata compile cannot resolve (publishing fails) #329

Found while verifying #95: publishAllToMavenLocal fails at :python-multiplatform:compileNativeMainKotlinMetadata with 'Unresolved reference pthread_mutex_init' in PlatformMutex.native.kt (#117, #302). The per-target compiles (iosSimulatorArm64, androidNativeArm64) pass, so the regression was not caught; the shared nativeMain metadata (iOS + androidNative) only sees commonized POSIX, and pthread_mutex_init is not in it. Completion: compileNativeMainKotlinMetadata passes; HandleTable stays mutually exclusive on native; per-target compiles and the HandleTable concurrency test pass.