Optimize edge lookup railnetwork (#2493)

## Description:

This PR optimizes how the rail network looks up railroads connecting two
stations by introducing an O(1) neighbor→railroad map on `TrainStation`.
It also updates `getOrientedRailroad` and railroad deletion to use this
new API, avoiding repeated linear scans over all railroads attached to a
station.

### What changed

- **TrainStation neighbor→railroad index**
- Added `railroadByNeighbor: Map<TrainStation, Railroad>` to
`TrainStation` for quick edge lookup.
  - Kept `railroads: Set<Railroad>` for iteration and existing APIs.
  - Updated lifecycle methods to keep both data structures in sync:
    - `addRailroad(railRoad: Railroad)` now:
      - Adds to `railroads`.
- Computes the neighbor station (`railRoad.from === this ? railRoad.to :
railRoad.from`).
      - Stores the mapping in `railroadByNeighbor`.
    - `removeRailroad(railRoad: Railroad)` now:
      - Removes from `railroads`.
      - Removes the corresponding entry from `railroadByNeighbor`.
- `clearRailroads()` now clears both `railroads` and
`railroadByNeighbor`.
- Added `getRailroadTo(station: TrainStation): Railroad | null` to
retrieve the connecting railroad in O(1).

- **Use the new API in `TrainStation` and `Railroad`**
- `TrainStation.removeNeighboringRails(station)` now calls
`removeRailroad(toRemove)` instead of manually deleting from the set,
ensuring the map stays in sync.
- `Railroad.delete(game)` now calls `from.removeRailroad(this)` and
`to.removeRailroad(this)` instead of mutating the sets directly.

- **Refactor `getOrientedRailroad` to use O(1) lookup**
- Replaced a linear scan over `from.getRailroads()` with a direct
lookup:

    ```ts
    export function getOrientedRailroad(
      from: TrainStation,
      to: TrainStation,
    ): OrientedRailroad | null {
      const railroad = from.getRailroadTo(to);
      if (!railroad) return null;
// If tiles are stored from -> to, we go forward when railroad.to === to
      const forward = railroad.to === to;
      return new OrientedRailroad(railroad, forward);
    }
    ```

  - Behavior is preserved:
- `getRailroadTo` returns the same `Railroad` instance that was
previously found by scanning `getRailroads()`.
- Direction (`forward` vs reversed) is still derived from the
`Railroad.from` / `.to` fields in the same way as before.

### Motivation

- `getOrientedRailroad` and upcoming logic both need to resolve “the
railroad between station A and station B” frequently.
- The old pattern (`for (const railroad of from.getRailroads()) { ...
}`) was:
  - O(degree) per lookup,
  - Repeated in multiple places,
- Harder to maintain as more features (like fare-based costs) touch this
code.
- Centralizing edge lookup in a dedicated `railroadByNeighbor` map makes
this:
  - **O(1)** per lookup,
  - Less error-prone (one source of truth),
- Easier to reuse from new systems (e.g. train pathfinding, fare-aware
logic).

### Impact / Risk

- **Public behavior:** No functional change in how railroads are
created, deleted, or oriented; only the lookup mechanism changed.
- **Internal invariants:** Correctness relies on:
- All railroad creations using `addRailroad` on both endpoints (already
true via `RailNetworkImpl.connect`).
- All removals (`Railroad.delete`,
`TrainStation.removeNeighboringRails`, `disconnectFromNetwork`) using
`removeRailroad` / `clearRailroads`, which this PR updates.
- **Tests:** Existing `TrainStation` tests still pass; they exercise
`addRailroad`, `removeNeighboringRails`, and `getRailroads()`, which
continue to behave the same from the outside.

## 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:

DISCORD_USERNAME

src/core/game/Railroad.ts

@@ -20,8 +20,8 @@ export class Railroad {
       isActive: false,
       railTiles,
     });
-    this.from.getRailroads().delete(this);
-    this.to.getRailroads().delete(this);
+    this.from.removeRailroad(this);
+    this.to.removeRailroad(this);
   }
 }
 
@@ -29,14 +29,11 @@ export function getOrientedRailroad(
   from: TrainStation,
   to: TrainStation,
 ): OrientedRailroad | null {
-  for (const railroad of from.getRailroads()) {
-    if (railroad.from === to) {
-      return new OrientedRailroad(railroad, false);
-    } else if (railroad.to === to) {
-      return new OrientedRailroad(railroad, true);
-    }
-  }
-  return null;
+  const railroad = from.getRailroadTo(to);
+  if (!railroad) return null;
+  // If tiles are stored from -> to, we go forward when railroad.to === to
+  const forward = railroad.to === to;
+  return new OrientedRailroad(railroad, forward);
 }
 
 /**

src/core/game/TrainStation.ts

@@ -76,6 +76,8 @@ export class TrainStation {
     {};
   private cluster: Cluster | null;
   private railroads: Set<Railroad> = new Set();
+  // Quick lookup from neighboring station to connecting railroad
+  private railroadByNeighbor: Map<TrainStation, Railroad> = new Map();
 
   constructor(
     private mg: Game,
@@ -91,10 +93,19 @@ export class TrainStation {
 
   clearRailroads() {
     this.railroads.clear();
+    this.railroadByNeighbor.clear();
   }
 
   addRailroad(railRoad: Railroad) {
     this.railroads.add(railRoad);
+    const neighbor = railRoad.from === this ? railRoad.to : railRoad.from;
+    this.railroadByNeighbor.set(neighbor, railRoad);
+  }
+
+  removeRailroad(railRoad: Railroad) {
+    this.railroads.delete(railRoad);
+    const neighbor = railRoad.from === this ? railRoad.to : railRoad.from;
+    this.railroadByNeighbor.delete(neighbor);
   }
 
   removeNeighboringRails(station: TrainStation) {
@@ -111,7 +122,7 @@ export class TrainStation {
         isActive: false,
         railTiles,
       });
-      this.railroads.delete(toRemove);
+      this.removeRailroad(toRemove);
     }
   }
 
@@ -139,6 +150,10 @@ export class TrainStation {
     return this.railroads;
   }
 
+  getRailroadTo(station: TrainStation): Railroad | null {
+    return this.railroadByNeighbor.get(station) ?? null;
+  }
+
   setCluster(cluster: Cluster | null) {
     this.cluster = cluster;
   }