feat: model immutable combat state

tests/Unit/Domain/CombatEngineTest.php

@@ -0,0 +1,32 @@
+<?php
+
+declare(strict_types=1);
+
+namespace BattleForge\Tests\Unit\Domain;
+
+use BattleForge\Domain\Battlefield;
+use BattleForge\Domain\MatchState;
+use BattleForge\Domain\Position;
+use BattleForge\Domain\UnitState;
+use PHPUnit\Framework\TestCase;
+
+final class CombatEngineTest extends TestCase
+{
+    public function testMatchAndUnitStateAreImmutable(): void
+    {
+        $unit = $this->unit('unit-1', 'alpha', new Position(0, 0));
+        $match = new MatchState(new Battlefield(8, 8), [$unit], 'alpha');
+
+        $moved = $unit->moveTo(new Position(1, 0))->spendAction();
+        $next = $match->withUnit($moved);
+
+        self::assertSame('0:0', $unit->position->key());
+        self::assertSame('1:0', $next->unit('unit-1')->position->key());
+        self::assertSame(1, $next->unit('unit-1')->actionsRemaining);
+    }
+
+    private function unit(string $id, string $team, Position $position): UnitState
+    {
+        return new UnitState($id, $team, $position, 10, 10, 4, 2, 4, 2);
+    }
+}