feat(applet): StepUpApplet derives session keys on SELECT when armed

After AUTH1 parks StepUpSK in SessionContext, the reader issues SELECT
to the Step-Up AID per spec §10.2. StepUpApplet.select() now picks up
the parked SK, derives StepUpSKDevice / StepUpSKReader via the
§8.4.3 HKDF (already implemented in AliroCrypto.deriveStepUpSessionKeys),
and stages both in transient CLEAR_ON_DESELECT fields for the ENVELOPE
(M1C.1) and EXCHANGE (M1B.1) handlers.

Introduces CryptoSingletons -- a lazy package-private holder for the
single AliroCrypto instance shared between AliroApplet and StepUpApplet.
Saves ~352 B of transient (kdfWorkbuf + hkdfPrevT + expandScratch)
versus a per-applet duplicate. Java Card forbids new in <clinit> so
the singleton uses lazy null-check init. Opt 1 prelude per
docs/plans/2026-06-11-step-up-implementation-v2.md.

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

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

@@ -0,0 +1,29 @@
+package com.dangerousthings.aliro;
+
+/**
+ * Lazy holders for crypto objects shared between {@link AliroApplet} and
+ * {@link StepUpApplet}. Both applets live in the same package and only one
+ * is selected at a time, so the shared instances are safe — Java Card runs
+ * one applet at a time and the JCRE serializes APDU dispatch.
+ *
+ * <p>Java Card forbids {@code new} in {@code <clinit>}, so the getter lazy-
+ * creates on first call. Subsequent calls return the cached instance. The
+ * combined transient footprint of one shared {@link AliroCrypto} is roughly
+ * 352 B (kdfWorkbuf 64 + hkdfPrevT 32 + expandScratch 256); without this
+ * sharing each applet would allocate its own.
+ */
+final class CryptoSingletons {
+
+    private static AliroCrypto aliroCrypto;
+
+    private CryptoSingletons() { }
+
+    /** Returns the process-wide {@link AliroCrypto} instance. Allocates on
+     *  first call; cheap thereafter. */
+    static AliroCrypto getAliroCrypto() {
+        if (aliroCrypto == null) {
+            aliroCrypto = new AliroCrypto();
+        }
+        return aliroCrypto;
+    }
+}