Render rats inside internal tunnel passages

Previously Rat.draw() hid any rat whose center fell inside a tunnel.
The clipping helper only handled single-entrance cells and returned None
for internal passages/crossroads, causing rats to vanish entirely.

Now internal tunnel cells (those with 0 or 2+ open sides) draw the rat
normally so it remains visible while walking through the tunnel.
Single-entrance cells keep the partial clip effect.
Also fix the visible-ratio math for DOWN/RIGHT clipping: the old
formulas subtracted cell_size from a local coordinate, producing
zero or negative visibility.

engine/graphics.py

@@ -347,9 +347,9 @@ class Graphics:
                         if below:
                             if left:
                                 if right:
-                                    # Internal tunnel passage: draw a neutral floor cap
-                                    # so the grey background does not show through.
-                                    draw_cave(random_wall_texture(), px + half_cell, py + half_cell, None)
+                                    # Internal tunnel passage: leave it empty so it uses
+                                    # the background fill color and stays visually open.
+                                    pass
                                 else:
                                     draw_cave(self.caves["RIGHT"], px, py, "RIGHT")
                             else: