feat(once): add Once::new_completed constructor

[SAY-5]

Closes #441.

Adds Once::new_completed(), a const constructor that produces a Once already in the Done state. Naming follows @Amanieu's suggestion in the issue (new_completed for consistency with the standard library's Once::is_completed). Existing Once::new() semantics are unchanged.

static INIT: Once = Once::new_completed();
assert_eq!(INIT.state(), OnceState::Done);
INIT.call_once(|| unreachable!()); // closure is skipped

Tests cover the new constructor and a regression test that Once::new() still runs its closure.