Fog of War improvements

The legacy Fog of War had hard, clipped edges where the visible area was revealed, and the brightness difference between the center and the outer area did not blend naturally, which clashed with the game's dark battlefield mood.

Given the nature of RTS, Fog of War is not just a vision-restriction feature; it is an important visual element that lets the player intuitively distinguish information they currently have from unexplored regions.
So the work needed to preserve functional readability while expressing a dark, natural look that matches the art direction.

To improve this, the center of the visible area stays clearly visible, and the structure was changed to a gradient that darkens stepwise toward the outer edge.
In addition, the previously visible clipping band was removed so the Fog boundary doesn't appear like a layer pasted on top of the screen but blends naturally with the background.