v0.4.0 — V30MZ validated against the V20 oracle

V30MZ validated against the V20 single-step hardware oracle.

cpu-v30mz was run against the V20 single-step oracle over ~620k runnable cases: 93.49% exact, zero defined-behaviour bugs — every divergence is a V20-only instruction or an officially-undefined flag, not a bug in the documented behaviour.

The run found and fixed the Flags::to_word high-bits bug, which also resolved the MD-bit open question (→ 0xF002). See docs/VALIDATION.md.