Summary
Add a mock Payment SCA credential profile as a new doctype and include it in testapp’s generated test documents, as part of Design Document Phase 1 implementation.

Design Doc Alignment
This work is part of Design Document Phase 1, specifically this section:
Phase 1 section

Problem / Motivation
There is no mock Digital Payment Credential in current test data, making payment-oriented testing and demos harder.

Scope

• Add new doctype definition: DigitalPaymentCredential
• Doctype identifier: org.multipaz.payment.sca.1
• Add sample/canned requests for minimal and full data
• Wire doctype into testapp provisioning list
• Add payment card art for the credential
• Add/extend unit test coverage for doctype registration/structure

Expected Behavior
After creating test documents in testapp, a payment credential card is visible in Document Store as:
Erika's Payment Card Credential

Acceptance Criteria

• Doctype exists and is discoverable in repository
• Provisioning includes payment credential
• Card art displays in testapp Document Store
• Unit tests pass for new doctype path
• Manual verification on Android and iPhone confirms visibility in Document Store

Test Evidence

• Unit test command:
  ./gradlew :multipaz-doctypes:jvmTest --tests "org.multipaz.documenttype.TestDocumentTypeRepository.testDigitalPaymentCredentialDocumentType"
• Manual test command used during validation:
  ./gradlew :samples:testapp:installBlueDebug
• Manual result: payment card visible in Document Store on Android and iPhone
• Video proof: https://bit.ly/4kOLYPY

Notes / Non-goals

• This is mock/test data, not production payment issuance.
• README updates are optional unless we want to explicitly advertise this doctype in docs.