nx_checkers_test.nx

nx_checkers_test.nx -- C1a substrate smoke. 8 assertions covering the initial setup at minimum complexity: 1. new game has 24 pieces total (12 red + 12 black) 2. red men occupy rows 5,6,7 dark squares (4 per row = 12 total) 3. black men occupy rows 0,1,2 dark squares 4. no pieces on rows 3 or 4 (the gap) 5. no pieces on any LIGHT square (parity invariant) 6. turn = RED (red moves first per standard rules) 7. outcome = ONGOING, force_continue = -1, plies = 0 8. coord helpers: is_dark + in_bounds + sq round-trip