perf: eliminate double map lookups, dynamic_cast in render loops, div by 255

- Replace count()+operator[] double lookups with find() or try_emplace()
  in gameObjectInstances_, playerTextureSlotsByModelId_, onlinePlayerAppearance_
- Add Entity::isUnit() helper; replace 5 dynamic_cast<Unit*> in per-frame
  UI rendering (nameplates, combat text, pet frame) with isUnit()+static_cast
- Add constexpr kInv255 reciprocal for per-pixel normal map generation loops
  in character_renderer and wmo_renderer

include/game/entity.hpp

@@ -153,6 +153,9 @@ public:
     ObjectType getType() const { return type; }
     void setType(ObjectType t) { type = t; }
 
+    /// True if this entity is a Unit or Player (both derive from Unit).
+    bool isUnit() const { return type == ObjectType::UNIT || type == ObjectType::PLAYER; }
+
     // Fields (for update values)
     void setField(uint16_t index, uint32_t value) {
         fields[index] = value;