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/configuration/DefaultConfig.ts

@@ -301,6 +301,21 @@ export class DefaultConfig implements Config {
   tradeShipSpawnRate(numberOfPorts: number): number {
     return Math.min(50, Math.round(10 * Math.pow(numberOfPorts, 0.6)));
   }
+  trainSpawnRate(numberOfStations: number): number {
+    return Math.round(50 * Math.pow(numberOfStations, 0.8));
+  }
+  trainGold(): Gold {
+    return BigInt(10_000);
+  }
+  trainStationMinRange(): number {
+    return 15;
+  }
+  trainStationMaxRange(): number {
+    return 80;
+  }
+  railroadMaxSize(): number {
+    return 100;
+  }
 
   unitInfo(type: UnitType): UnitInfo {
     switch (type) {
@@ -450,11 +465,33 @@ export class DefaultConfig implements Config {
           constructionDuration: this.instantBuild() ? 0 : 2 * 10,
           upgradable: true,
         };
+      case UnitType.Factory:
+        return {
+          cost: (p: Player) =>
+            p.type() === PlayerType.Human && this.infiniteGold()
+              ? 0n
+              : BigInt(
+                  Math.min(
+                    1_000_000,
+                    Math.pow(
+                      2,
+                      p.unitsIncludingConstruction(UnitType.Factory).length,
+                    ) * 125_000,
+                  ),
+                ),
+          territoryBound: true,
+          constructionDuration: this.instantBuild() ? 0 : 2 * 10,
+        };
       case UnitType.Construction:
         return {
           cost: () => 0n,
           territoryBound: true,
         };
+      case UnitType.Train:
+        return {
+          cost: () => 0n,
+          territoryBound: false,
+        };
       default:
         assertNever(type);
     }