pub const TAIL: u32 = u32::MAX; // 4_294_967_295u32Expand description
The “no next” sentinel stored in a slot’s link to denote the BOTTOM of the
stack (the first index pushed onto an empty stack chains to this).
u32::MAX.
TAIL (u32::MAX) terminates a link chain; the head’s empty sentinel is
INDEX_MASK (at most 0xFFFF). Push and pop translate between them.