refactor: extract findOnUseSpellId helper, add warden hash comment

- spell_handler: extract duplicated item on-use spell lookup into
  findOnUseSpellId() — was copy-pasted in useItemBySlot and useItemInBag
- warden_handler: add why-comment explaining the door model HMAC-SHA1
  hash table (wall-hack detection for unmodified 3.3.5a client data)

include/game/spell_handler.hpp

@@ -249,6 +249,9 @@ private:
     void handleChannelUpdate(network::Packet& packet);
 
     // --- Internal helpers ---
+    // Find the on-use spell for an item (trigger=0 Use or trigger=5 NoDelay).
+    // CMSG_USE_ITEM requires a valid spellId or the server silently ignores it.
+    uint32_t findOnUseSpellId(uint32_t itemId) const;
     void loadSpellNameCache() const;
     void loadSkillLineAbilityDbc();
     void categorizeTrainerSpells();