Nations: Fix city farming + reactive defense posts + fix nuked territory capture 🛡️ (#3814)

## Description:

Would be very good to get these fixes last minute into v31.

- **Farming nations for cities is fixed**: Here you can see city farming
in action, vari farms Finland:
https://www.youtube.com/watch?v=J4J0ajlnSHs&t=137s - Lots of 125k gold
cities... Now nations will build defense posts instead of cities:

- **Nations now build defense posts reactively** instead of through the
normal structure build order. When a nation is under significant land
attack (incoming/own troops >= 35%), it places a defense post near the
attack front -- skipped on Easy, capped at 1 on Medium, and scaled by
the incoming troop ratio on Hard/Impossible. Posts spread along the
front. The old `defensePostValue()` placement path is removed.

- **Nations now capture nuked territory.**
`hasLandBorderWithTerraNullius()` was incorrectly filtering out tiles
with fallout, causing the `nuked` attack strategy to never dispatch a
attack. Big bug

.

- [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
- [X] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced

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

FloPinguin

src/core/execution/utils/AiAttackBehavior.ts

@@ -755,11 +755,7 @@ export class AiAttackBehavior {
   private hasLandBorderWithTerraNullius(): boolean {
     for (const border of this.player.borderTiles()) {
       for (const neighbor of this.game.neighbors(border)) {
-        if (
-          this.game.isLand(neighbor) &&
-          !this.game.hasOwner(neighbor) &&
-          !this.game.hasFallout(neighbor)
-        ) {
+        if (this.game.isLand(neighbor) && !this.game.hasOwner(neighbor)) {
           return true;
         }
       }