macOS: `dispatch_semaphore_t` leak in `semabin` destructor

[zhengying]

semabin::~semabin() on macOS calls release() but never dispatch_release(), so every watch instance leaks a dispatch_semaphore_t.

Current code (detail/wtr/watcher/semabin.hpp):

inline ~semabin() noexcept { this->release(); }

release() only signals the semaphore to wake up waiters. It does not balance the +1 retain from dispatch_semaphore_create(0).

Fix:

inline ~semabin() noexcept
{
    this->release();
    dispatch_release(this->sem);
}

The project is built as pure C++ (no .mm files, no ARC), so dispatch_release is required here. No #if guards needed for the current build setup.

Impact: Any program that creates and destroys multiple watch objects on macOS will slowly leak GCD semaphores.