Normalize near-white pixels to pure white in all loaded assets

Assets like lose.png contain thousands of (254,254,254) pixels on a
background that is also white. When drawn on a white panel these
near-white pixels are slightly darker, producing a visible gray seam.

Apply a global normalization in load_image(): any opaque pixel with
R=G=B >= 250 is clamped to (255,255,255). Applied to every asset, not
only those with transparent_color, so the background is always pure
white regardless of off-by-one in the source PNG.

engine/sdl2.py

@@ -318,6 +318,22 @@ class GameWindow:
             ]
             image.putdata(new_data)
 
+        # Normalize near-white background pixels to pure white to avoid
+        # dark seams when the texture is drawn on a white panel. Applied
+        # to every asset, not only those with transparent_color, so the
+        # background is always pure white.
+        if image.mode != "RGBA":
+            image = image.convert("RGBA")
+        datas = image.getdata()
+        normalized = [
+            (255, 255, 255, item[3]) if item[3] > 0
+                and item[0] >= 250 and item[1] >= 250 and item[2] >= 250
+                and item[0] == item[1] == item[2]
+            else item
+            for item in datas
+        ]
+        image.putdata(normalized)
+
         # Scale image: tiles are now 64px (was 20px), multiply by 5/8 to reach cell_size (40px)
         image = image.resize((image.width * 5 // 8, image.height * 5 // 8), Image.NEAREST)