A thread-safe utility class that returns the `once` value once ever, and then the `other` value from then on.

OnceThenOther.cs

public sealed class OnceThenOther<T>(T once, T other)
{
	private Int32 flag = 0;
	
	// Checking the flag value normally before exchanging it atomically is an optimization with two parts:
	// A) Contentious first access will all synchronize on the atomic exchange
	// B) Subsequent accesses will see the local cached value of `1`, so they won't have to perform the heavier atomic exchange
	public T Value => flag == 0 && Interlocked.Exchange(ref flag, 1) == 0 ? once : other;
}