Show rail ghost for initial factory 🚂 (#4294)

## Problem

Fixes #4284. When you build a factory in an area with **no pre-existing
factory** (e.g. just a city nearby), no rail ghost preview appeared —
even though building the factory *would* lay rail lines connecting it to
that city.

## Root cause

`computeGhostRailPaths` in `RailNetworkImpl.ts` had two factory-hostile
assumptions:

1. It bailed out early unless a `Factory` was already in range
(`hasUnitNearby(..., UnitType.Factory)`).
2. It only matched neighbors that were *already* train stations
(`findStation(...)` → skipped if null).

But a **Factory** always becomes a station itself and *promotes* nearby
City/Port/Factory into the rail network (see `FactoryExecution`). So it
needs no pre-existing factory, and its neighbors won't be stations yet
on first build. A **City/Port** only joins the network when a factory
already exists (`CityExecution`/`PortExecution`) — so their behavior is
correctly left unchanged.

## Fix

- Skip the "factory must be nearby" gate when the placed unit is itself
a `Factory`.
- For a factory build, pathfind to nearby City/Port/Factory even if they
aren't stations yet. City/Port keep connecting only to existing
stations.

## Tests

Added two cases to `RailNetwork.test.ts` (factory connects with no
pre-existing factory; city still doesn't without one). All 25 tests
pass.

## Note on scope

As @Katokoda noted on the issue, a fully build-exact preview
(neighboring structures also connecting to *each other*, merging
existing networks, etc.) is larger and order-dependent. This PR resolves
the reported bug — the initial factory now shows its rail ghost — and
leaves the exact-match cascade as a separate follow-up.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

src/core/game/RailNetworkImpl.ts

@@ -253,8 +253,15 @@ export class RailNetworkImpl implements RailNetwork {
     const minRangeSquared = this.game.config().trainStationMinRange() ** 2;
     const maxPathSize = this.game.config().railroadMaxSize();
 
-    // Cannot connect if outside the max range of a factory
-    if (!this.game.hasUnitNearby(tile, maxRange, UnitType.Factory)) {
+    // A City or Port only joins the rail network when a Factory is already in
+    // range (see CityExecution/PortExecution). A Factory always becomes a
+    // station and pulls nearby City/Port/Factory into the network itself, so
+    // it needs no pre-existing factory to connect to.
+    const buildingFactory = unitType === UnitType.Factory;
+    if (
+      !buildingFactory &&
+      !this.game.hasUnitNearby(tile, maxRange, UnitType.Factory)
+    ) {
       return [];
     }
 
@@ -273,22 +280,34 @@ export class RailNetworkImpl implements RailNetwork {
       if (neighbor.distSquared <= minRangeSquared) continue;
 
       const neighborStation = this._stationManager.findStation(neighbor.unit);
-      if (!neighborStation) continue;
 
-      const alreadyReachable = connectedStations.some(
-        (s) =>
-          this.distanceFrom(
-            neighborStation,
-            s,
-            this.maxConnectionDistance - 1,
-          ) !== -1,
-      );
-      if (alreadyReachable) continue;
+      // Building a factory connects to nearby structures even if they aren't
+      // stations yet — they get promoted to stations when the factory is
+      // built. For a city/port, only existing stations are relevant.
+      let targetTile: TileRef;
+      if (neighborStation) {
+        const alreadyReachable = connectedStations.some(
+          (s) =>
+            this.distanceFrom(
+              neighborStation,
+              s,
+              this.maxConnectionDistance - 1,
+            ) !== -1,
+        );
+        if (alreadyReachable) continue;
+        targetTile = neighborStation.tile();
+      } else if (buildingFactory) {
+        targetTile = neighbor.unit.tile();
+      } else {
+        continue;
+      }
 
-      const path = this.pathService.findTilePath(tile, neighborStation.tile());
+      const path = this.pathService.findTilePath(tile, targetTile);
       if (path.length > 0 && path.length < maxPathSize) {
         paths.push(path);
-        connectedStations.push(neighborStation);
+        if (neighborStation) {
+          connectedStations.push(neighborStation);
+        }
       }
     }