Add impassable terrain 🗺️ (#4340)

## Description:

Relates to #3725

Adds a new **Impassable** terrain type that enables non-rectangular maps
and creates impassable barriers on the map. Painted with pure black
(`#000`) in the map editor's `image.png`.

**Encoding:** Impassable terrain is encoded in the binary format as
`isLand=1, magnitude=31` (previously unused). The Go map generator
detects `#000` pixels and produces this encoding. The map generator's
minimap downscaling gives impassable highest priority (Impassable >
Water > Land). Thumbnails render impassable as transparent so the map
picker background shows through.

**Rendering:** Impassable tiles render as the map background colour
(`rgb(60, 60, 60)`, matching `gl.clearColor` in `Renderer.ts`), making
them visually indistinguishable from the area outside the map quad. This
enables maps to appear non-rectangular.

**Gameplay restrictions:** Impassable terrain cannot be:
- Owned (`conquer()` throws)
- Attacked (`AttackExecution` skips impassable tiles in both `tick()`
and `addNeighbors()`)
- Nuked (targeting rejected in `nukeSpawn()`, blast radius filtered in
`tilesToDestroy()`)
- Spawned on (nations, human players, and structures all reject
impassable tiles)
- Converted to water (guarded in `WaterManager` and `setWater()`)

**Nuke trajectories:** Nuke trajectories cannot cross impassable
terrain, matching the existing map-border enforcement. This is checked
at launch time in `NukeExecution.tick()`. The client-side trajectory
preview turns red with a red X where the arc crosses impassable terrain
(reusing the existing SAM-intercept visual pipeline in
`NukeTrajectory.ts`). The nuke ghost preview is completely hidden when
hovering over impassable terrain (same as hovering outside the map).


https://github.com/user-attachments/assets/ff131146-9749-41e0-892a-617e5cd16c54

Impassable terrain is transparent on the thumbnail:

<img width="213" height="152" alt="Screenshot 2026-06-18 211640"
src="https://github.com/user-attachments/assets/ede16f8c-9239-4ab1-be5d-0ba81cce5e9e"
/>

Tested with water nukes, made sure there is no water depth gradient near
the impassable terrain, just like at the world border:

<img width="774" height="771" alt="Screenshot 2026-06-18 212348"
src="https://github.com/user-attachments/assets/4429069d-911b-48e8-91e3-7307d42c9397"
/>

Models used: GLM 5.2 and MiMo 2.5 Pro 😄

## Please complete the following:

- [X] I have added screenshots for all UI updates
- [X] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [X] I have added relevant tests to the test directory

## Please put your Discord username so you can be contacted if a bug or
regression is found:

FloPinguin

resources/maps/losangeles/manifest.json

@@ -3,17 +3,17 @@
   "id": "LosAngeles",
   "map": {
     "height": 2276,
-    "num_land_tiles": 2066156,
+    "num_land_tiles": 2055581,
     "width": 1800
   },
   "map16x": {
     "height": 569,
-    "num_land_tiles": 123167,
+    "num_land_tiles": 122366,
     "width": 450
   },
   "map4x": {
     "height": 1138,
-    "num_land_tiles": 508420,
+    "num_land_tiles": 505680,
     "width": 900
   },
   "multiplayer_frequency": 8,