feat: resolve attacks and elimination victory

src/Domain/CombatEngine.php

@@ -12,11 +12,7 @@ final class CombatEngine
     {
         $this->assertMatchActive($match);
 
-        try {
-            $unit = $match->unit($unitId);
-        } catch (InvalidArgumentException $exception) {
-            throw new CombatException("Unknown unit: {$unitId}.", previous: $exception);
-        }
+        $unit = $this->unitOrFail($match, $unitId);
         $this->assertCanAct($match, $unit);
 
         $occupied = [];
@@ -40,6 +36,56 @@ final class CombatEngine
         return $next->copy(actionLog: $actionLog);
     }
 
+    public function attack(MatchState $match, string $attackerId, string $targetId): MatchState
+    {
+        $this->assertMatchActive($match);
+
+        $attacker = $this->unitOrFail($match, $attackerId);
+        $this->assertCanAct($match, $attacker);
+
+        if ($attacker->hasAttacked) {
+            throw new CombatException('Unit has already attacked this turn.');
+        }
+
+        $target = $this->unitOrFail($match, $targetId);
+
+        if ($target->teamId === $attacker->teamId || $target->isDefeated()) {
+            throw new CombatException('Target must be an active enemy unit.');
+        }
+
+        $distance = abs($attacker->position->x - $target->position->x)
+            + abs($attacker->position->y - $target->position->y);
+
+        if ($distance !== 1) {
+            throw new CombatException('Target is outside attack range.');
+        }
+
+        $terrainDefense = $match->battlefield->terrainAt($target->position)->defenseBonus();
+        $damage = max(1, $attacker->attack - $target->defense - $terrainDefense);
+        $updatedAttacker = $attacker->markAttacked()->spendAction();
+        $updatedTarget = $target->takeDamage($damage);
+        $next = $match->withUnit($updatedAttacker)->withUnit($updatedTarget);
+        $actionLog = [...$match->actionLog, "{$attacker->id} attacked {$target->id} for {$damage} damage"];
+        $next = $next->copy(actionLog: $actionLog);
+
+        foreach ($next->units as $unit) {
+            if ($unit->teamId !== $attacker->teamId && !$unit->isDefeated()) {
+                return $next;
+            }
+        }
+
+        return $next->withWinner($attacker->teamId);
+    }
+
+    private function unitOrFail(MatchState $match, string $unitId): UnitState
+    {
+        try {
+            return $match->unit($unitId);
+        } catch (InvalidArgumentException $exception) {
+            throw new CombatException("Unknown unit: {$unitId}.", previous: $exception);
+        }
+    }
+
     private function assertMatchActive(MatchState $match): void
     {
         if ($match->winnerTeamId !== null) {