Cache shared-water computation for nation port placement 💧 (#3696)

## Description:

- Cache `sharedWaterComponents` globally with a 30-tick (~3s) TTL so all
nations share one `O(total_border_tiles)` pass instead of each nation
re-scanning every other player's border on every call.
- Treat ocean as always-shared: any ocean neighbor short-circuits as a
valid port site, skipping the `getWaterComponent` lookup in both the
build pass and the per-tile port check.
- Exclude bots and mutually-embargoed players from the trade-partner
candidate set, so nations no longer avoid port sites that only "share"
water with a player they can never trade with.

Port placement is not time-critical, so the 3-second staleness is
acceptable and lets the expensive build amortize across many attack
cycles.

### Performance
Benchmarked on World map (2000×1000, 61 nations) with the realistic call
pattern of ~3 nations invoking `sharedWaterComponents` per tick:

- **Before (main):** ~414 μs per tick
- **After:** ~8 μs per tick amortized (29/30 ticks hit the warm cache;
1/30 rebuilds)
- **~50× faster** on this AI hot path

## 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/NationStructureBehavior.ts

@@ -108,7 +108,7 @@ export class NationStructureBehavior {
           Math.floor(this.player.numTilesOwned() / TILES_PER_CITY_EQUIVALENT),
         )
       : this.player.unitsOwned(UnitType.City);
-    this._sharedWaterComponents = this.sharedWaterComponents();
+    this._sharedWaterComponents = this.game.sharedWaterComponents(this.player);
     const hasCoastalTiles = this._sharedWaterComponents !== null;
 
     // Build order for non-city structures (priority order)
@@ -167,39 +167,6 @@ export class NationStructureBehavior {
     return false;
   }
 
-  /**
-   * Returns the set of water components shared with at least one other player,
-   * or null if there are none.
-   */
-  private sharedWaterComponents(): Set<number> | null {
-    // Collect all water-component IDs reachable from this player's coast.
-    const playerComponents = new Set<number>();
-    for (const tile of this.player.borderTiles()) {
-      if (!this.game.isShore(tile)) continue;
-      for (const neighbor of this.game.neighbors(tile)) {
-        if (!this.game.isWater(neighbor)) continue;
-        const comp = this.game.getWaterComponent(neighbor);
-        if (comp !== null) playerComponents.add(comp);
-      }
-    }
-    if (playerComponents.size === 0) return null;
-
-    // Keep only components that at least one other player also touches.
-    const shared = new Set<number>();
-    for (const other of this.game.players()) {
-      if (other === this.player) continue;
-      for (const tile of other.borderTiles()) {
-        if (!this.game.isShore(tile)) continue;
-        for (const neighbor of this.game.neighbors(tile)) {
-          if (!this.game.isWater(neighbor)) continue;
-          const comp = this.game.getWaterComponent(neighbor);
-          if (comp !== null && playerComponents.has(comp)) shared.add(comp);
-        }
-      }
-    }
-    return shared.size > 0 ? shared : null;
-  }
-
   /**
    * Determines if we should build more of this structure type based on
    * the current city count and the configured ratio.
@@ -507,6 +474,9 @@ export class NationStructureBehavior {
       if (shared === null) return false;
       for (const neighbor of this.game.neighbors(t)) {
         if (!this.game.isWater(neighbor)) continue;
+        // Ocean is always considered shared, so any ocean neighbor makes the
+        // tile a valid port site — skip the component lookup.
+        if (this.game.isOcean(neighbor)) return true;
         const comp = this.game.getWaterComponent(neighbor);
         if (comp !== null && shared.has(comp)) return true;
       }

src/core/execution/nation/SharedWaterCache.ts

@@ -0,0 +1,99 @@
+import { Game, Player, PlayerType } from "../../game/Game";
+
+/**
+ * Cache for "which water components does each nation share with a
+ * valid trade partner". Used by nation AI to decide whether to spend cycles
+ * trying to place a port on a given coastline.
+ *
+ * Rebuilt at most once every TTL_TICKS (3s at 10 ticks/s). Port placement is
+ * not time-critical - a nation noticing a newly-valid port site a few seconds
+ * late is fine and lets us amortize the O(total_border_tiles) build across
+ * far more callers than a per-tick cache would.
+ */
+const TTL_TICKS = 30;
+
+/** Sentinel added to a player's shared-water set to signal "touches ocean". */
+const OCEAN_SENTINEL = -1;
+
+export class SharedWaterCache {
+  private tick: number = -Infinity;
+  private byPlayer: Map<Player, Set<number> | null> | null = null;
+
+  constructor(private game: Game) {}
+
+  get(player: Player): Set<number> | null {
+    const tick = this.game.ticks();
+    if (this.byPlayer === null || tick - this.tick >= TTL_TICKS) {
+      this.byPlayer = this.build();
+      this.tick = tick;
+    }
+    return this.byPlayer.get(player) ?? null;
+  }
+
+  private build(): Map<Player, Set<number> | null> {
+    const game = this.game;
+
+    // Pass 1: for each non-bot player, record which water bodies they touch
+    // and which lakes have them as a candidate trade partner. Bots are skipped
+    // entirely — nation AI is the only caller, and bots are never candidate
+    // trade partners.
+    const playerToWater = new Map<
+      Player,
+      { hasOcean: boolean; lakes: Set<number> }
+    >();
+    const lakePartners = new Map<number, Player[]>();
+
+    for (const player of game.players()) {
+      if (player.type() === PlayerType.Bot) continue;
+
+      let hasOcean = false;
+      const lakes = new Set<number>();
+      for (const tile of player.borderTiles()) {
+        if (!game.isShore(tile)) continue;
+        for (const neighbor of game.neighbors(tile)) {
+          if (!game.isWater(neighbor)) continue;
+          if (game.isOcean(neighbor)) {
+            hasOcean = true;
+            continue;
+          }
+          const comp = game.getWaterComponent(neighbor);
+          if (comp !== null) lakes.add(comp);
+        }
+      }
+      playerToWater.set(player, { hasOcean, lakes });
+
+      for (const c of lakes) {
+        let arr = lakePartners.get(c);
+        if (arr === undefined) {
+          arr = [];
+          lakePartners.set(c, arr);
+        }
+        arr.push(player);
+      }
+    }
+
+    // Pass 2: ocean is treated as always shared (nation AI short-circuits on
+    // ocean neighbors). Lake components are shared only if some *other* player
+    // on that component can trade with P (i.e. no mutual embargo).
+    const result = new Map<Player, Set<number> | null>();
+    for (const [player, { hasOcean, lakes }] of playerToWater) {
+      const shared = new Set<number>();
+
+      if (hasOcean) shared.add(OCEAN_SENTINEL);
+
+      for (const c of lakes) {
+        const partners = lakePartners.get(c);
+        if (partners === undefined) continue;
+        for (const other of partners) {
+          if (other !== player && player.canTrade(other)) {
+            shared.add(c);
+            break;
+          }
+        }
+      }
+
+      result.set(player, shared.size > 0 ? shared : null);
+    }
+    return result;
+  }
+}