Integer Key Formatting for PK/SK

When integer types are used as PK (Partition Key) or SK (Sort Key) fields, they are zero-padded to their maximum decimal digit width so that lexicographic (string) sort order matches numeric sort order.

Unsigned types

Signed types (offset encoding)

Signed integers are converted to unsigned by adding an offset equal to |T::MIN| (i.e. 2^(bits-1)), then zero-padded to the same width as the corresponding unsigned type. This maps the full signed range onto 0..=U::MAX while preserving numeric order.

usize is always treated as u64, and isize as i64.