Remove SIGURG blocking from maybe_grow_with to restore preemptive scheduling on grown stacks

[Copilot]

maybe_grow_with was blocking SIGURG for the entire duration of corosensei::on_stack(), which completely disables preemptive scheduling while on a grown stack. Any dead loop inside the callback becomes uninterruptible.

• Removed SIGURG thread_block/thread_set_mask around on_stack() call
• Simplified OnStackGuard to only handle stack_infos cleanup (still needed for unwind safety)

Before:

let mut sigurg_mask = SigSet::empty();
sigurg_mask.add(Signal::SIGURG);
let old_mask = SigSet::thread_get_mask();
_ = sigurg_mask.thread_block();
// callback runs here with SIGURG blocked — preemption dead
let r = corosensei::on_stack(stack, callback);
// guard restores mask on drop

After:

co.stack_infos_mut().push_back(StackInfo::from(&stack));
let guard = OnStackGuard { stack_infos: co.stack_infos_mut() };
let r = corosensei::on_stack(stack, callback);
drop(guard);