Opt-in marker for `nuctest --category selfcheck` (pure-nucleos system
self-consistency: `nucleos traces` cover → `simulate --check-trace`,
no dzn golden). Excluded from the default `all` gate — the full cover
is tens of thousands of traces. Validated 2026-06-19: Robot 248/248,
BallSorter 13196/13196, LegoBallSorter 10482/10482 (0 bails).
