feat(editor): selected NPC marker is larger and yellow-cyan tinted

Marker geometry now reacts to npc.selected: 2.5x base radius (vs 1.5x),
saturated yellow with cyan tinge, and full alpha. Marker rebuild also
fires on selection change so the highlight appears immediately rather
than only after the next placement.

tools/editor/editor_app.hpp

@@ -175,6 +175,7 @@ public:
 private:
     size_t lastObjCount_ = 0;
     size_t lastNpcCount_ = 0;
+    int lastNpcSelIdx_ = -1;
     EditorMode mode_ = EditorMode::Sculpt;
     float waterHeight_ = 100.0f;
     uint16_t waterType_ = 0;