feat: add impassable terrain for non-rectangular maps

map-generator/README.md

@@ -86,6 +86,16 @@ If you are doing work in image editing software or using automated tools, `./map
 - `Pixel` -> `Terrain Type & Magnitude` mapping in `GenerateMap`
 - `Terrain Type` -> `Thumbnail Color` mapping in `getThumbnailColor`
 
+### Impassable Terrain
+
+Pure black pixels (`#000000` / `rgb(0, 0, 0)` with alpha ≥ 20) are encoded as **impassable terrain**. This is a solid, static void that:
+
+- Cannot be owned, attacked, or nuked.
+- Nuke trajectories cannot pass over it (just as they cannot leave the map border).
+- Renders as the map background colour, making the map appear non-rectangular.
+
+Use impassable terrain to carve out non-rectangular map shapes or to create barriers that divide regions without water.
+
 In-Game, terrain is rendered using themes. The color of a tile is determined dynamically based on
 its **Terrain Type** and **Magnitude**. Theme Files: