chore(applet): drop M1 stub comments + dead paths + naming consistency (M2F.2)

Sweep stale M1-era cruft and milestone task markers obsoleted by M2:

- StepUpApplet class Javadoc: fix stale @link target for
  finalizeAccessDocument (now takes verifier+scratch65), drop "TODO"
  prose for shipped M2A.3 verify-at-finalize, rewrite "INS_EXCHANGE stub
  was retired" past-tense note as present-tense ownership statement.
- StepUpApplet: drop "M1B / M1C" / "M2E.1" / "M2E.2" / "M2D.3+M2D.4"
  task markers from comments and method Javadocs; describe current
  behavior, not milestone provenance.
- StepUpApplet: rewrite "decrypt-and-discard" sink Javadoc — that sink
  now also stages the EXCHANGE Reader Status request plaintext.
- StepUpSession: drop "matching M1 behaviour" Javadoc trailers.
- AliroApplet bitmap comment: align with the new method name and drop
  the M1 historical aside that's now adjacent to the only behavior left.
- AliroAppletTest: rewrite "Once M1B.1 / M1C.1 land..." as present tense.
- CredentialStore.markAccessDocumentVerified() → ForTesting suffix to
  match the sibling markAccessDocumentFinalizedForTesting() and add an
  explanatory Javadoc clarifying that production callers reach the
  verified state via finalizeAccessDocument(short, CoseVerifier, byte[]).
  Two test call sites updated.

No behavior change. Tests: 147 / 0 / 3 pre-existing GCM errors. Net
diff +46/-44 = +2 LOC (the rename's explanatory Javadoc minus the cruft).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

applet/src/test/java/com/dangerousthings/aliro/CredentialStoreSerializationTest.java

@@ -97,7 +97,7 @@ class CredentialStoreSerializationTest {
     void accessDocumentVerifiedRoundTripsThroughSerialize() {
         CredentialStore s1 = CredentialStore.bootstrap();
         s1.resetForTesting();
-        s1.markAccessDocumentVerified();
+        s1.markAccessDocumentVerifiedForTesting();
 
         RecordingSink buf = new RecordingSink();
         s1.writeTo(buf);