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).
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