Perf spawn train (#3130)

## Description:

Train spawning hot-path optimization (trade destination selection)

## Summary
This PR reduces per-tick overhead in train spawning by removing
temporary allocations and reducing work in the
destination-selection path.

The change focuses on `Cluster` trade destination lookup and how
`TrainStationExecution` picks a destination.

## What changed
### 1) Maintain a “trade-capable” station subset per cluster
`src/core/game/TrainStation.ts`

- `Cluster` now maintains:
  - `stations`: all stations in the cluster (unchanged)
- `tradeStations`: maintained subset of stations that can act as trade
endpoints (`City` or `Port`)
- `tradeStations` is kept in sync in:
  - `addStation()`
  - `removeStation()`
  - `clear()`

Impact:
- Trade queries no longer scan every station in the cluster; they only
scan `tradeStations`.

### 2) Add cheap eligibility helpers
`src/core/game/TrainStation.ts`

- `hasAnyTradeDestination(player)`:
- Fast early-exit check: returns as soon as it finds any eligible trade
destination.
- `randomTradeDestination(player, random)`:
- Picks a random eligible trade destination directly without
materializing an intermediate `Set`.

### 3) Use reservoir sampling for single-pass random choice
`src/core/game/TrainStation.ts`

`Cluster.randomTradeDestination()` uses reservoir sampling:
- Iterates `tradeStations` once.
- Maintains a running count of eligible stations (`eligibleSeen`).
- Replaces the selected station with probability `1/eligibleSeen`.

Properties:
- Uniform selection among eligible stations.
- One pass instead of “count then pick by index” (two pass).
- Allocation-free.
- Returns `null` when no eligible destination exists.

### 4) Update train spawning to avoid temporary sets
`src/core/execution/TrainStationExecution.ts`

- Previously: `spawnTrain()` called `cluster.availableForTrade()` and
then `random.randFromSet(...)`.
  - This built a new `Set` on the hot path.
- Now:
  - Early-exit via `cluster.hasAnyTradeDestination(owner)`.
  - Destination via `cluster.randomTradeDestination(owner, random)`.

Net effect:
- Less per-tick work and no per-spawn temporary `Set` allocations.

## Why this helps
Train spawning happens frequently and can become a hot path in large
games / large rail clusters.
Avoiding repeated allocations and reducing work inside `tick()` helps
keep frame/update time predictable.

## notes
- Trade rules are unchanged (`tradeAvailable(player)` still gates
eligibility).
- Destination selection remains random-uniform over eligible
`City`/`Port` stations that satisfy `tradeAvailable(player)`.
- `TrainStationExecution` now avoids calling `spawnTrain()` entirely
when `spawnTrains` is falsy (it was already guarded inside).




## Please complete the following:

- [ ] I have added screenshots for all UI updates
- [ ] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [ ] I have added relevant tests to the test directory
- [ ] 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

tests/core/game/Cluster.test.ts

@@ -1,9 +1,13 @@
 import { vi, type Mocked } from "vitest";
+import { UnitType } from "../../../src/core/game/Game";
 import { Cluster, TrainStation } from "../../../src/core/game/TrainStation";
 
 const createMockStation = (id: string): Mocked<TrainStation> => {
   return {
     id,
+    unit: {
+      type: vi.fn(() => UnitType.City),
+    } as any,
     setCluster: vi.fn(),
     getCluster: vi.fn(() => null),
   } as any;

tests/core/game/RailNetwork.test.ts

@@ -1,4 +1,4 @@
-import { Unit } from "../../../src/core/game/Game";
+import { Unit, UnitType } from "../../../src/core/game/Game";
 import {
   RailNetworkImpl,
   StationManagerImpl,
@@ -14,6 +14,7 @@ const createMockStation = (unitId: number): any => {
     unit: {
       id: unitId,
       setTrainStation: vi.fn(),
+      type: vi.fn(() => UnitType.City),
     },
     tile: vi.fn(),
     neighbors: vi.fn(() => []),