feat(editor): terrain holes, recent textures, sculpt panel polish

- Punch Hole / Fill Hole buttons in Sculpt panel: creates terrain
  holes (4x4 bitmask) for cave entrances, mine shafts, etc.
  Uses brush radius to determine affected area.
- Recent Textures: paint panel shows last 6 used textures as quick-
  select buttons (no need to re-search the full list)
- Holes saved in ADT format (MCNK holes field) and respected by
  the mesh generator (triangles skipped at hole positions)

tools/editor/terrain_editor.hpp

@@ -55,6 +55,10 @@ public:
     void setWaterLevel(const glm::vec3& center, float radius, float waterHeight, uint16_t liquidType = 0);
     void removeWater(const glm::vec3& center, float radius);
 
+    // Hole editing (4x4 bitmask per chunk — cave entrances, mine shafts)
+    void punchHole(const glm::vec3& center, float radius);
+    void fillHole(const glm::vec3& center, float radius);
+
     bool hasUnsavedChanges() const { return dirty_; }
     void markSaved() { dirty_ = false; }