Constant UNBOUNDED_MEASURING_PAD 

const UNBOUNDED_MEASURING_PAD: usize = _ /* 9_223_372_036_854_775_807usize */;

Padding width that puts every wire length a varint can express within measuring reach.

Half of usize::MAX, so adding the buffered bytes cannot overflow PaddedBuf::remaining; a varint tops out at 2^62 - 1, well inside it on a 64-bit target.