Optimize nuke explosion (#3176)

## Description:

When a nuke detonates, the explosion is looping over each tiles. Among
other things, it filters the owner units to find the `TransportShips`
units.

This operation is cpu-intensive and repeated over each tiles can make
the tick noticably slower.

On this screenshot the detonation function took 100ms:
<img width="1617" height="488" alt="image"
src="https://github.com/user-attachments/assets/07009b18-4342-4caf-9e82-9ae5147b63f8"
/>
<img width="1645" height="375" alt="image"
src="https://github.com/user-attachments/assets/fe9ead87-550a-4166-96ab-092d0c08be82"
/>

I suspect it led to a few frame freeze when I MIRVed too.

Suggestion: loop over the impacted players rather than the tiles.

With this suggestion, the same nuke takes between 4ms to 20ms:
<img width="989" height="365" alt="image"
src="https://github.com/user-attachments/assets/25c0faf0-cc34-41b7-8091-b14bde6db595"
/>


However this changes the death formula used, as they were repeated over
each tiles with ever-smaller values, and with this change the operation
is done all-at-once. This will result in a different outcome.

In my opinion the performance gain is seductive enough to maybe tweak
the formula to make it work with this revised strategy.
What do you think?

## Please complete the following:

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

IngloriousTom

src/core/game/UnitImpl.ts

@@ -164,7 +164,7 @@ export class UnitImpl implements Unit {
   }
 
   setTroops(troops: number): void {
-    this._troops = troops;
+    this._troops = Math.max(0, troops);
   }
   troops(): number {
     return this._troops;