feat: define CredentialStore serialization contract

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>

applet/src/main/java/com/dangerousthings/aliro/SerializationSink.java

@@ -0,0 +1,23 @@
+package com.dangerousthings.aliro;
+
+/**
+ * Minimal write-side abstraction matching the shape of the GlobalPlatform
+ * Amendment H {@code org.globalplatform.upgrade.Element} write API. The
+ * AMD-H adapter (added in a later epic, J3R452-only) is a one-class bridge
+ * that implements this interface against the real {@code Element}.
+ *
+ * <p>Keeping the interface in the main source tree means
+ * {@link CredentialStore#writeTo(SerializationSink)} can be authored,
+ * tested, and locked down today without pulling in the AMD-H package, which
+ * only exists on JCOP 4.5 / J3R452-class cards.
+ *
+ * <p>Element semantics: once a byte[] reference is written, the storage
+ * provider treats it as immutable from the caller's perspective. Test
+ * fakes that buffer writes should make defensive copies to mirror this.
+ */
+interface SerializationSink {
+    SerializationSink write(byte b);
+    SerializationSink write(short s);
+    SerializationSink write(boolean z);
+    SerializationSink write(byte[] arr);
+}