4fb87b6200
Move the four StepUp session-state fields (StepUpSKDevice, StepUpSKReader, stepup_reader_counter, stepup_device_counter) and the IV-stamp + counter-advance math out of StepUpApplet into a dedicated StepUpSession holder. Adds a unit-testable surface for the spec §8.3.1.6/8/9 IV layout and §8.4.3 counter-advance behaviour (4 new tests pinning reader/device IV layout, carry across byte 2, and the 32-bit BE wrap at 0xFFFFFFFF). Behaviour-preserving: full StepUpAppletTest stays green with no test-assertion changes (the test-only getters keep their signatures and just delegate to session.skDevice / session.skReader). StepUpApplet shrinks 444 -> 394 LOC. Full suite: 138 tests, 0 failures, 3 errors (pre-existing GCM trio).