Improve nations 🤖 (#3206)

## Description:

- `AiAttackBehavior`: Because bots delete stolen structures now, nations
prioritize attacking bots with structures
- `NationMIRVBehavior`: Nations no longer MIRV enemies who already got
MIRVed in the last 30 seconds. Some humans complained about getting
double-MIRVed by nations. And in games with very high starting gold, ALL
nations MIRVed the same player (stop steamroll logic).
- `NationAllianceBehavior`: Fixes a comparison logic bug (Thanks to
Deshack)
- `NationNukeBehavior.ts`: Little atom bomb perceived cost balance
change
- `MIRVExecution`: To make sure the MIRVing nations are attacking the
MIRVed nations (even if they don't share a border), the relation gets
updated in both directions now.
- `SinglePlayerModal` & `HostLobbyModal`: Update the default difficulty
to "Medium" (to synchronize the defaults with the public game default)

## 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:

FloPinguin

src/core/execution/nation/NationAllianceBehavior.ts

@@ -87,7 +87,7 @@ export class NationAllianceBehavior {
       }
       return false;
     }
-    // Reject if otherPlayer has allied with 50% or more of all players (Hard and Impossible only)
+    // Reject if otherPlayer has allied with a lot of players (Hard and Impossible only)
     // To make sure there are enough non-friendly players in the game to stop the crown with nukes
     if (this.hasTooManyAlliances(otherPlayer)) {
       return false;
@@ -148,7 +148,7 @@ export class NationAllianceBehavior {
       .filter((p) => p.type() !== PlayerType.Bot).length;
     const otherPlayerAlliances = otherPlayer.alliances().length;
 
-    if (difficulty !== Difficulty.Hard) {
+    if (difficulty === Difficulty.Hard) {
       return otherPlayerAlliances >= totalPlayers * 0.5;
     } else {
       return otherPlayerAlliances >= totalPlayers * 0.25;

src/core/execution/nation/NationMIRVBehavior.ts

@@ -4,7 +4,9 @@ import {
   Game,
   Gold,
   Player,
+  PlayerID,
   PlayerType,
+  Tick,
   UnitType,
 } from "../../game/Game";
 import { TileRef } from "../../game/GameMap";
@@ -18,7 +20,15 @@ import {
   respondToMIRV,
 } from "./NationEmojiBehavior";
 
+// 30 seconds at 10 ticks/second
+const MIRV_COOLDOWN_TICKS = 300;
+
 export class NationMIRVBehavior {
+  // Shared across all NationMIRVBehavior instances.
+  // Tracks the last tick a MIRV was sent at each player, so multiple nations don't pile-on the same target.
+  // Especially important for games with very high starting gold settings.
+  private static recentMirvTargets = new Map<PlayerID, Tick>();
+
   constructor(
     private random: PseudoRandom,
     private game: Game,
@@ -119,19 +129,19 @@ export class NationMIRVBehavior {
     }
 
     const inboundMIRVSender = this.selectCounterMirvTarget();
-    if (inboundMIRVSender) {
+    if (inboundMIRVSender && !this.wasRecentlyMirved(inboundMIRVSender)) {
       this.maybeSendMIRV(inboundMIRVSender);
       return true;
     }
 
     const victoryDenialTarget = this.selectVictoryDenialTarget();
-    if (victoryDenialTarget) {
+    if (victoryDenialTarget && !this.wasRecentlyMirved(victoryDenialTarget)) {
       this.maybeSendMIRV(victoryDenialTarget);
       return true;
     }
 
     const steamrollStopTarget = this.selectSteamrollStopTarget();
-    if (steamrollStopTarget) {
+    if (steamrollStopTarget && !this.wasRecentlyMirved(steamrollStopTarget)) {
       this.maybeSendMIRV(steamrollStopTarget);
       return true;
     }
@@ -223,6 +233,17 @@ export class NationMIRVBehavior {
     return null;
   }
 
+  // MIRV Cooldown Methods
+  private wasRecentlyMirved(target: Player): boolean {
+    const lastTick = NationMIRVBehavior.recentMirvTargets.get(target.id());
+    if (lastTick === undefined) return false;
+    return this.game.ticks() - lastTick < MIRV_COOLDOWN_TICKS;
+  }
+
+  private recordMirvHit(target: Player): void {
+    NationMIRVBehavior.recentMirvTargets.set(target.id(), this.game.ticks());
+  }
+
   // MIRV Helper Methods
   private getValidMirvTargetPlayers(): Player[] {
     if (this.player === null) throw new Error("not initialized");
@@ -261,6 +282,7 @@ export class NationMIRVBehavior {
     const centerTile = this.calculateTerritoryCenter(enemy);
     if (centerTile && this.player.canBuild(UnitType.MIRV, centerTile)) {
       this.game.addExecution(new MirvExecution(this.player, centerTile));
+      this.recordMirvHit(enemy);
       this.emojiBehavior.sendEmoji(AllPlayers, EMOJI_NUKE);
       respondToMIRV(this.game, this.random, enemy);
     }

src/core/execution/nation/NationNukeBehavior.ts

@@ -659,8 +659,8 @@ export class NationNukeBehavior {
     this.recentlySentNukes.push([tick, tile, nukeType]);
     if (nukeType === UnitType.AtomBomb) {
       this.atomBombsLaunched++;
-      // Increase perceived cost by 35% each time to simulate saving up for a MIRV (higher than hydro to make atom bombs less attractive for the lategame)
-      this.atomBombPerceivedCost = (this.atomBombPerceivedCost * 135n) / 100n;
+      // Increase perceived cost by 50% each time to simulate saving up for a MIRV (higher than hydro to make atom bombs less attractive for the lategame)
+      this.atomBombPerceivedCost = (this.atomBombPerceivedCost * 150n) / 100n;
     } else if (nukeType === UnitType.HydrogenBomb) {
       this.hydrogenBombsLaunched++;
       // Increase perceived cost by 25% each time to simulate saving up for a MIRV