Fix numeric digit landing in decimal area when typed before prefix (#2615)

[thorn0]

Summary

When the cursor is placed before the prefix on a numeric mask with radixPoint, the DOM position translates past the buffer end, causing _isValid to fall through to alternation switching and place the digit in the decimal area (e.g. typing "5" produced "$0.5" instead of "$5").

• Detect this case in preValidation and redirect to the integer part when the field has no entered digits
• The check ignores positions populated by checkVal (generatedInput) and the negation symbol, so the fix also applies after typing "-" alone

Closes #2615, closes #1367

Test plan

• Added 12 QUnit tests covering:
  • cursor at position 0 with empty field
  • cursor at 0 after typing -
  • cursor at 0 after deleting all content
  • digitsOptional: false variant
  • non-regression: cursor at 0 when digits already present
  • multi-digit typing at position 0
  • alternative radixPoint/groupSeparator
  • full-width digit input
  • multi-character prefix ("$ ") with cursor at 0 and mid-prefix
  • multi-digit non-regression with existing value

AI disclosure

This PR was written primarily by Claude Code. Follow-up fixes were reviewed and amended by Codex.