feat(applet): StepUpApplet INS_EXCHANGE decrypt-and-discard with 9000 ack

X-CUBE-ALIRO firmware sends a Reader Status sub-event via EXCHANGE
(CLA=0x80, INS=0xC9) after the step-up AID SELECT, per Aliro §8.3.3.5
Table 8-14. The payload is encrypted with StepUpSKReader using AES-256-GCM
with IV = 0x0000000000000000 || stepup_reader_counter (4B BE) per §8.3.1.8.

For Milestone 1 we decrypt-and-discard: tag verification proves we have
matching session keys, then we return SW=9000 with empty payload. M2
will add a proper encrypted Reader Status response sub-event.

Extends CryptoSingletons to hold the shared AliroGcm instance too --
opt 1 sibling of the AliroCrypto sharing from M1A.2. Adds AliroGcm.decrypt
since the prior pipeline only encrypted (AUTH1 response path).

Counter starts at 1 per session-bound init (spec §8.4.3 -> mdoc [6]
§9.1.1.5), incremented after each successful decrypt.

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

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

@@ -15,6 +15,7 @@ package com.dangerousthings.aliro;
 final class CryptoSingletons {
 
     private static AliroCrypto aliroCrypto;
+    private static AliroGcm aliroGcm;
 
     private CryptoSingletons() { }
 
@@ -26,4 +27,19 @@ final class CryptoSingletons {
         }
         return aliroCrypto;
     }
+
+    /** Returns the process-wide {@link AliroGcm} instance. Same Java-Card
+     *  {@code <clinit>}-cannot-{@code new} rationale as {@link #getAliroCrypto()}:
+     *  lazy-allocate on first call so both {@link AliroApplet} and
+     *  {@link StepUpApplet} share one userland-GCM machine. Each shared
+     *  instance saves ~370 B of transient/EEPROM footprint that a per-applet
+     *  duplicate would otherwise pay. The two applets are never selected
+     *  simultaneously and the JCRE serializes APDU dispatch, so the shared
+     *  scratch and {@code AESKey} slot don't race. */
+    static AliroGcm getAliroGcm() {
+        if (aliroGcm == null) {
+            aliroGcm = new AliroGcm();
+        }
+        return aliroGcm;
+    }
 }