feat: add GameMap and A* pathfinding with tests

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/map/pathfinding.ts

@@ -0,0 +1,81 @@
+import type { Position } from '@dflike/shared';
+import type { GameMap } from './GameMap.js';
+
+interface Node {
+  x: number;
+  y: number;
+  g: number;
+  h: number;
+  f: number;
+  parent: Node | null;
+}
+
+function heuristic(a: Position, b: Position): number {
+  return Math.abs(a.x - b.x) + Math.abs(a.y - b.y);
+}
+
+const NEIGHBORS = [
+  { dx: 0, dy: -1 },
+  { dx: 0, dy: 1 },
+  { dx: -1, dy: 0 },
+  { dx: 1, dy: 0 },
+];
+
+export function findPath(map: GameMap, start: Position, goal: Position): Position[] | null {
+  if (start.x === goal.x && start.y === goal.y) return [];
+
+  const open: Node[] = [];
+  const closed = new Set<string>();
+  const key = (x: number, y: number) => `${x},${y}`;
+
+  const startNode: Node = {
+    x: start.x, y: start.y,
+    g: 0, h: heuristic(start, goal), f: heuristic(start, goal),
+    parent: null,
+  };
+  open.push(startNode);
+
+  while (open.length > 0) {
+    // Find lowest f
+    let bestIdx = 0;
+    for (let i = 1; i < open.length; i++) {
+      if (open[i].f < open[bestIdx].f) bestIdx = i;
+    }
+    const current = open.splice(bestIdx, 1)[0];
+
+    if (current.x === goal.x && current.y === goal.y) {
+      // Reconstruct path (excluding start)
+      const path: Position[] = [];
+      let node: Node | null = current;
+      while (node && !(node.x === start.x && node.y === start.y)) {
+        path.push({ x: node.x, y: node.y });
+        node = node.parent;
+      }
+      return path.reverse();
+    }
+
+    closed.add(key(current.x, current.y));
+
+    for (const { dx, dy } of NEIGHBORS) {
+      const nx = current.x + dx;
+      const ny = current.y + dy;
+      if (!map.isWalkable(nx, ny) || closed.has(key(nx, ny))) continue;
+
+      const g = current.g + 1;
+      const h = heuristic({ x: nx, y: ny }, goal);
+      const existing = open.find(n => n.x === nx && n.y === ny);
+
+      if (existing) {
+        if (g < existing.g) {
+          existing.g = g;
+          existing.f = g + h;
+          existing.parent = current;
+        }
+      } else {
+        open.push({ x: nx, y: ny, g, h, f: g + h, parent: current });
+      }
+    }
+  }
+
+  return null; // No path found
+}