Adds in-tree SerializationSink/Source interfaces matching the shape of
the AMD-H Element API, plus writeTo/readFrom on CredentialStore and a
round-trip test. The AMD-H adapter (added in a later epic) just bridges
Element -> these interfaces, so the field layout is testable today
without depending on a JCOP 4.5 image.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
