Constant CPU_SNAPSHOT_VERSION 

pub const CPU_SNAPSHOT_VERSION: u8 = 3;

Schema version for the CPU snapshot blob.

• v1 (v0.9.0 ..): registers + interrupt latches + cycle bookkeeping.
• v2 (W3-Stage-4 promotion, 2026-06-10): appends the master-clock substrate pipeline — master_clock (u64) + the mc_need_nmi / mc_prev_need_nmi / mc_run_irq / mc_prev_run_irq / mc_prev_nmi_line latches (1 byte each).
• v3 (v2.0.0 “Timebase” rc.1, ADR 0028): the byte layout is IDENTICAL to v2 — cycles and master_clock are both still written, unchanged. What changes is the guarantee: as of the beta.1–beta.4 one-clock promote, cycles is no longer an independently-tracked counter (it is assigned from Bus::cycle_count() at every start_cycle, see cpu.rs), so a v3 blob’s cycles/master_clock pair is guaranteed internally consistent by construction in a way a pre-promote v1/v2 blob was only coincidentally consistent (kept in sync by parallel increments, not derivation). The version bump exists to make that distinction an explicit, checked contract rather than an implicit assumption — see ADR 0028 for the full MAJOR-boundary decision. v1/v2 blobs are no longer upconverted; Cpu::restore rejects any version other than CPU_SNAPSHOT_VERSION (the caller-side Nes::restore_inner already enforced this via a strict per-section equality check before this bump — the upconvert path removed here was dead code, unreachable through the only real caller).