Add trains (#1159)

## Description:

Add a rail network to handle train stations/railroad between structures.

Changes:
- `RailNetwork` is responsible for the train station graph. Use it to
connect new `TrainStations`
- A `RailRoad` connects two `TrainStation`
- No loop possible in the rail network
- Train stations handles its railroads
- Added a layer to draw the railroads under the structures

#### Clusters
- To speed up computations, each `TrainStation` references its own
cluster
- A cluster is a list of `TrainStation` connected with each other,
created by the `RailNetwork` when connecting the station
- Train stations spawn trains randomly depending on its current cluster
size
- A `TrainStation` decides randomly of the train destination by picking
one from the cluster

#### Production building:
- Added a factory which has no gameplay impact currently. _To be
discussed._

#### Train stops:
- When a train reaches a factory, it's filled with a "cargo". The loaded
trains has no impact currently. _To be discussed._
- When a train reaches a city, the player earn 10k gold
- When a train reaches a port, it sends a new tradeship if possible
- If a destination/source is destroyed, the train & railroad are deleted
too


https://github.com/user-attachments/assets/42375c17-9e04-4a42-98d0-708c81ffd609


https://github.com/user-attachments/assets/fbecdb53-a516-4df8-87fb-1f9a62c4efa0



## 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
- [x] I understand that submitting code with bugs that could have been
caught through manual testing blocks releases and new features for all
contributors

## Please put your Discord username so you can be contacted if a bug or
regression is found:

IngloriousTom

---------

Co-authored-by: Scott Anderson <scottanderson@users.noreply.github.com>

src/core/game/UnitImpl.ts

@@ -4,6 +4,7 @@ import {
   MessageType,
   Player,
   Tick,
+  TrainType,
   Unit,
   UnitInfo,
   UnitType,
@@ -28,9 +29,12 @@ export class UnitImpl implements Unit {
   private _troops: number;
   private _missileTimerQueue: number[] = [];
   private _readyMissileCount: number = 1;
+  private _hasTrainStation: boolean = false;
   private _patrolTile: TileRef | undefined;
   private _level: number = 1;
   private _targetable: boolean = true;
+  private _loaded: boolean | undefined;
+  private _trainType: TrainType | undefined;
 
   constructor(
     private _type: UnitType,
@@ -53,6 +57,9 @@ export class UnitImpl implements Unit {
       "patrolTile" in params ? (params.patrolTile ?? undefined) : undefined;
     this._targetUnit =
       "targetUnit" in params ? (params.targetUnit ?? undefined) : undefined;
+    this._loaded =
+      "loaded" in params ? (params.loaded ?? undefined) : undefined;
+    this._trainType = "trainType" in params ? params.trainType : undefined;
 
     switch (this._type) {
       case UnitType.Warship:
@@ -123,6 +130,9 @@ export class UnitImpl implements Unit {
       missileTimerQueue: this._missileTimerQueue,
       readyMissileCount: this._readyMissileCount,
       level: this.level(),
+      hasTrainStation: this._hasTrainStation,
+      trainType: this._trainType,
+      loaded: this._loaded,
     };
   }
 
@@ -351,6 +361,15 @@ export class UnitImpl implements Unit {
     return this._level;
   }
 
+  setTrainStation(trainStation: boolean): void {
+    this._hasTrainStation = trainStation;
+    this.mg.addUpdate(this.toUpdate());
+  }
+
+  hasTrainStation(): boolean {
+    return this._hasTrainStation;
+  }
+
   increaseLevel(): void {
     this._level++;
     if ([UnitType.MissileSilo, UnitType.SAMLauncher].includes(this.type())) {
@@ -358,4 +377,19 @@ export class UnitImpl implements Unit {
     }
     this.mg.addUpdate(this.toUpdate());
   }
+
+  trainType(): TrainType | undefined {
+    return this._trainType;
+  }
+
+  isLoaded(): boolean | undefined {
+    return this._loaded;
+  }
+
+  setLoaded(loaded: boolean): void {
+    if (this._loaded !== loaded) {
+      this._loaded = loaded;
+      this.mg.addUpdate(this.toUpdate());
+    }
+  }
 }