Perf/small refactor: NationStructureBehavio (#3237)

## Description:

PR 5/x in effort to break up PR
https://github.com/openfrontio/OpenFrontIO/pull/3220. Follows on already
merged https://github.com/openfrontio/OpenFrontIO/pull/3236.

Please see if these can be merged for v30.

**NationStructureBehavior**:
- maybeSpawnStructure: cache this.game to be used twice.
- maybeSpawnStructure: instead of hardcoded ruling out Defense Post for
upgrade check, check dynamically if type is upgradable. That way if
defense posts ever do become upgradable, we don't run into a bug right
away.

- maybeUpgradeStructure: removed canUpgradeUnit check. Since it already
checked this right before in findBestStructureToUpgrade, so only
upgradable units are returned. And canUpgradeUnit is also checked right
after in UpgradeStructureExecution. So we're going from 3 times to 2
times canUpgradeUnit, small perf win too.

- findBestStructureToUpgrade: cache this.game to be used thrice.

- shouldBuildStructure: cache this.game.config() to be used twice.

- getTotalStructureDensity: this.player.units can handle an array of
unit types to count. Input StructureTypes like this so we don't need a
loop and count, and only have to get an array length once.
getTotalStructureDensity needs to ignore unit levels so we can't make
use of other pre-defined functions in PlayerImpl (which were created to
avoid array length calls), but at least this saves a few.

## 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:

tryout33

src/core/execution/nation/NationStructureBehavior.ts

@@ -173,7 +173,8 @@ export class NationStructureBehavior {
     cityCount: number,
     hasCoastalTiles: boolean,
   ): boolean {
-    const { difficulty } = this.game.config().gameConfig();
+    const gameConfig = this.game.config();
+    const { difficulty } = gameConfig.gameConfig();
     const ratios = getStructureRatios(difficulty);
     const config = ratios[type];
     if (config === undefined) {
@@ -186,7 +187,7 @@ export class NationStructureBehavior {
     if (
       type === UnitType.Factory &&
       hasCoastalTiles &&
-      !this.game.config().isUnitDisabled(UnitType.Port)
+      !gameConfig.isUnitDisabled(UnitType.Port)
     ) {
       ratio *= FACTORY_COASTAL_RATIO_MULTIPLIER;
     }
@@ -219,6 +220,7 @@ export class NationStructureBehavior {
   }
 
   private maybeSpawnStructure(type: UnitType): boolean {
+    const game = this.game;
     const perceivedCost = this.getPerceivedCost(type);
     if (this.player.gold() < perceivedCost) {
       return false;
@@ -228,7 +230,7 @@ export class NationStructureBehavior {
     const structures = this.player.units(type);
     if (
       this.getTotalStructureDensity() > UPGRADE_DENSITY_THRESHOLD &&
-      type !== UnitType.DefensePost
+      game.config().unitInfo(type).upgradable
     ) {
       if (this.maybeUpgradeStructure(structures)) {
         return true;
@@ -249,7 +251,7 @@ export class NationStructureBehavior {
     if (canBuild === false) {
       return false;
     }
-    this.game.addExecution(new ConstructionExecution(this.player, type, tile));
+    game.addExecution(new ConstructionExecution(this.player, type, tile));
     return true;
   }
 
@@ -330,10 +332,8 @@ export class NationStructureBehavior {
       return false;
     }
     const structureToUpgrade = this.findBestStructureToUpgrade(structures);
-    if (
-      structureToUpgrade !== null &&
-      this.player.canUpgradeUnit(structureToUpgrade)
-    ) {
+    if (structureToUpgrade !== null) {
+      //canUpgradeUnit already checked in findBestStructureToUpgrade and again in UpgradeStructureExecution
       this.game.addExecution(
         new UpgradeStructureExecution(this.player, structureToUpgrade.id()),
       );
@@ -346,12 +346,10 @@ export class NationStructureBehavior {
    * Calculates total structure density across player's territory.
    */
   private getTotalStructureDensity(): number {
-    let totalStructures = 0;
-    for (const type of StructureTypes) {
-      totalStructures += this.player.units(type).length; // ignoring levels
-    }
     const tilesOwned = this.player.numTilesOwned();
-    return tilesOwned > 0 ? totalStructures / tilesOwned : 0;
+    return tilesOwned > 0
+      ? this.player.units(...StructureTypes).length / tilesOwned
+      : 0; //ignoring levels for structures
   }
 
   /**
@@ -359,6 +357,7 @@ export class NationStructureBehavior {
    * In 50% of cases, picks the second or third best to add variety.
    */
   private findBestStructureToUpgrade(structures: Unit[]): Unit | null {
+    const game = this.game;
     if (structures.length === 0) {
       return null;
     }
@@ -370,7 +369,7 @@ export class NationStructureBehavior {
     }
 
     // Based on difficulty, chance to just pick a random structure
-    const { difficulty } = this.game.config().gameConfig();
+    const { difficulty } = game.config().gameConfig();
     let randomChance: number;
     switch (difficulty) {
       case Difficulty.Easy:
@@ -403,9 +402,9 @@ export class NationStructureBehavior {
 
       // Check if protected by any SAM, using per-SAM level-based range
       for (const sam of samLaunchers) {
-        const samRange = this.game.config().samRange(sam.level());
+        const samRange = game.config().samRange(sam.level());
         const samRangeSquared = samRange * samRange;
-        const distSquared = this.game.euclideanDistSquared(
+        const distSquared = game.euclideanDistSquared(
           structure.tile(),
           sam.tile(),
         );