TUID parsing requires canonical 32-character hexadecimal strings

TUID parsing requires canonical 32-character hexadecimal strings

Tuid::from_str now accepts only exactly 32 ASCII hexadecimal characters (0-9, a-f, and A-F). Short inputs and inputs with a leading sign, which previously parsed as u128 values, now return ParseTuidError. The canonical 32-character form produced by Tuid::Display continues to roundtrip.

Use a zero-padded canonical string when constructing a Tuid from text:

let tuid: Tuid = "1".parse()?; // before
let tuid: Tuid = "00000000000000000000000000000001".parse()?; // after