Fix zero padding for localized integers

fmt::format("{:08L}", -12) produces 00000-12 even in the classic locale. The documented zero-padding rule requires -0000012, with zeros after the sign and any base prefix.

We now include numeric padding in the localized integer writer's output size and emit the zeros between the prefix and the grouped digits.

The regression test fails before the fix and passes afterward, and the existing formatting, header-only implementation and wide-character suites pass.

— ChipLab