Switches to Uint8 rejection sampling against the largest multiple of
the charset length that fits in a byte. Eliminates the ~16 ppm
overweight the previous `% N` over Uint32 would otherwise leave on the
first 38 chars. Adds a vitest distribution check.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
