Fix transport ship's troop count to update when a hydro hits the player. (#4381)

**Add approved & assigned issue number here:**

Resolves #4308 

## Description:

When nuclear damage reduces a player's troop count, it also affects any
transports ships in the water. This works well and is useful to avoid
exploiting tranports to avoid hydro damages.

`UnitImpl.setTroops()` changes the transports troop count without
queuing a unit update. the core value changes, but the client never
receives a fresh UnitUpdate unless something else touches the ship.

- UnitImpl.ts now emits a UnitUpdate when a unit troop count actually
changes.
- NukeExecution.ts batches transport ship nuke losses, then applies one
final troop update per ship.
- Attack.test.ts now asserts the nuke tick includes a transport
UnitUpdate with the reduced troop count.

## Please complete the following:

- [N/A] I have added screenshots for all UI updates
- [N/A] 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:

elliotlepley

src/core/game/UnitImpl.ts

@@ -172,7 +172,12 @@ export class UnitImpl implements Unit {
   }
 
   setTroops(troops: number): void {
-    this._troops = Math.max(0, troops);
+    const nextTroops = Math.max(0, troops);
+    if (this._troops === nextTroops) {
+      return;
+    }
+    this._troops = nextTroops;
+    this.mg.addUpdate(this.toUpdate());
   }
   troops(): number {
     return this._troops;