Iso revealed the WHOLE map from move 0: unexplored (dark) cells fell back to the floor tile
(_tileUrl(dark) -> tiles.floor) and rendered as floor. Now skip dark cells entirely (unexplored =
void), and dim explored tiles by the lightGrid brightness (globalAlpha, floor to 0.16) so the
remembered area fades to black with distance — parity with the 2D modes and 3D (which already
skips dark + uses THREE distance fog).