v3.9.1 — validateRequiredCharacteristics helper

New Features

• validateRequiredCharacteristics() — pure utility function that validates mandatory characteristics are present before card creation. Compares SubjectCharacteristic[] from getObjectCharc() against your CardCharacteristicInput[] and returns missing mandatory items. 9 unit tests.

Documentation

• Products module docs: added v3.9.0 section (SubjectCharacteristic types, isRequiredForCreate, code examples)
• Finances module docs: fixed method count (13→12, removed phantom legacy alias)
• README/CHANGELOG updated for v3.9.0
• TypeDoc regenerated with new utility function

Stats

• 2121 tests passing
• 0 TypeDoc warnings
• All validation tools clean

Full Changelog: v3.9.0...v3.9.1