feat: implement NPC systems (needs decay, brain, movement)

Add three ECS systems for NPC simulation:
- needsDecaySystem: decrements hunger/energy each tick, clamped at 0
- npcBrainSystem: goal selection based on need thresholds with pathfinding
- movementSystem: advances entities along computed paths

Includes 8 passing tests covering decay, goal prioritization, and movement.

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

server/src/systems/movementSystem.ts

@@ -0,0 +1,32 @@
+import { Direction, type Movement, type Position } from '@dflike/shared';
+import type { World } from '../ecs/World.js';
+
+function directionFromDelta(dx: number, dy: number): number {
+  if (Math.abs(dx) > Math.abs(dy)) return dx > 0 ? Direction.RIGHT : Direction.LEFT;
+  return dy > 0 ? Direction.DOWN : Direction.UP;
+}
+
+export function movementSystem(world: World): void {
+  for (const entity of world.query('position', 'movement')) {
+    const movement = world.getComponent<Movement>(entity, 'movement')!;
+    const pos = world.getComponent<Position>(entity, 'position')!;
+
+    if (movement.state !== 'walking' || movement.path.length === 0) {
+      if (movement.state === 'walking' && movement.path.length === 0) {
+        movement.state = 'idle';
+        movement.target = null;
+      }
+      continue;
+    }
+
+    const next = movement.path.shift()!;
+    movement.direction = directionFromDelta(next.x - pos.x, next.y - pos.y);
+    pos.x = next.x;
+    pos.y = next.y;
+
+    if (movement.path.length === 0) {
+      movement.state = 'idle';
+      movement.target = null;
+    }
+  }
+}