feat: add shared normalization helpers

[alyldas]

Summary:

• Added shared public input normalization for text-like values.
• Added explicit visible and length-preserving mask character normalization helpers while keeping normalizeMaskChar compatible.
• Covered nullish input, arbitrary values, empty masks, astral masks, and length-preserving masking behavior.

Tests:

• npm test
• npm run check (using a temporary npm cache outside the user home)

Do not merge until maintainer approval.

Closes #14