Gravity Four Drop Grid Showdown

Builds a complete two-player gravity-drop four-in-a-row game (6x7 board, real win detection on all 4 axes). Found and fixed a real bug via targeted unit testing: the win-check only looked one direction per axis, missing wins completed from the other end of the line. 16 unit tests plus 45 fully-automated random games through the real UI, 0 errors, every game reaches a result. (AI-assisted, hand-verified.)