feat(editor): NPC scatter tool, adjacent tile creation, multi-tile prep

- Scatter tool: place N creatures in a radius around cursor position
  with random rotation and uniform disk distribution
- File > Add Adjacent Tile: creates and exports a blank tile N/S/E/W
  of current (foundation for multi-tile zone editing)
- Scatter UI: count slider (1-30), radius slider (10-200)
- Scatter places all copies with same stats/behavior as template

tools/editor/npc_spawner.hpp

@@ -81,6 +81,10 @@ public:
     // Template creature for placement
     CreatureSpawn& getTemplate() { return template_; }
 
+    // Scatter: place multiple copies in a radius around a point
+    void scatter(const CreatureSpawn& base, const glm::vec3& center,
+                 float radius, int count);
+
 private:
     uint32_t nextId();
     std::vector<CreatureSpawn> spawns_;