fix(core): destroy defense posts on tile capture instead of downgrading and transferring ownership (#1563) (#4016)

## Description: 

Capturing defense posts previously demoted their level by 1, and
transferred ownership to the invading player if their level was still
above 0. The expected strategic behavior is that defense posts should
always be destroyed (deleted) upon capture.

This fix updates PlayerExecution.ts's structure tick loop to immediately
destroy the Defense Post unit via u.delete(true, captor) instead of
transferring ownership. It also rewrites the corresponding unit tests in
PlayerExecution.test.ts to verify the complete destruction of Defense
Posts of all levels (including level 2+) when the tile owner changes.

## 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
- [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:

barfires

tests/core/executions/PlayerExecution.test.ts

@@ -46,7 +46,7 @@ describe("PlayerExecution", () => {
     expect(game.unitCount(UnitType.DefensePost)).toBe(0);
   });
 
-  test("DefensePost lv. 2+ is downgraded when tile owner changes", () => {
+  test("DefensePost lv. 2+ is destroyed when tile owner changes", () => {
     const tile = game.ref(50, 50);
     player.conquer(tile);
     const defensePost = player.buildUnit(UnitType.DefensePost, tile, {});
@@ -60,11 +60,8 @@ describe("PlayerExecution", () => {
     otherPlayer.conquer(tile);
     executeTicks(game, 2);
 
-    expect(defensePost.level()).toBe(1);
-    expect(game.unitCount(UnitType.DefensePost)).toBe(1);
-    expect(otherPlayer.units(UnitType.DefensePost)).toHaveLength(1);
-    expect(defensePost.owner()).toBe(otherPlayer);
-    expect(defensePost.isActive()).toBe(true);
+    expect(game.unitCount(UnitType.DefensePost)).toBe(0);
+    expect(defensePost.isActive()).toBe(false);
   });
 
   test("Non-DefensePost structures are transferred (not downgraded) when tile owner changes", () => {