feat: model battlefield terrain and movement costs

src/Domain/Position.php

@@ -0,0 +1,24 @@
+<?php
+
+declare(strict_types=1);
+
+namespace BattleForge\Domain;
+
+final readonly class Position
+{
+    public function __construct(
+        public int $x,
+        public int $y,
+    ) {
+    }
+
+    public function key(): string
+    {
+        return $this->x . ':' . $this->y;
+    }
+
+    public function distanceTo(self $position): int
+    {
+        return abs($this->x - $position->x) + abs($this->y - $position->y);
+    }
+}