Add parser characterization tests and compatibility fixture

[rsheyd]

Summary

• add dependency-free characterization tests for the existing parser
• document repeated-name and serializeArray limitations without changing runtime behavior
• expand the manual fixture with non-sensitive standard and edge-case controls
• document how to run tests and load FormVault locally in Chrome

Why

The tests establish the current behavior before parser changes are considered. This makes later compatibility fixes easier to review and helps prevent accidental regressions.

Checks

• npm test
• git diff --check