Fix vanilla spell cast and use-item packet formats for Turtle/Classic

SpellCastTargets target mask is uint16 in vanilla 1.12.x, not uint32
like WotLK. The 2 extra bytes corrupted every spell packet. Also add
classic CMSG_USE_ITEM builder (bag+slot+spellIndex+targets only, no
spellId/itemGuid/glyphIndex/castFlags fields that WotLK added).

include/game/packet_parsers.hpp

@@ -50,6 +50,11 @@ public:
         return CastSpellPacket::build(spellId, targetGuid, castCount);
     }
 
+    /** Build CMSG_USE_ITEM (WotLK default: bag + slot + castCount + spellId + itemGuid + glyphIndex + castFlags + targets) */
+    virtual network::Packet buildUseItem(uint8_t bagIndex, uint8_t slotIndex, uint64_t itemGuid) {
+        return UseItemPacket::build(bagIndex, slotIndex, itemGuid);
+    }
+
     // --- Character Enumeration ---
 
     /** Parse SMSG_CHAR_ENUM */
@@ -238,6 +243,7 @@ public:
                                          const MovementInfo& info,
                                          uint64_t playerGuid = 0) override;
     network::Packet buildCastSpell(uint32_t spellId, uint64_t targetGuid, uint8_t castCount) override;
+    network::Packet buildUseItem(uint8_t bagIndex, uint8_t slotIndex, uint64_t itemGuid) override;
     bool parseCastFailed(network::Packet& packet, CastFailedData& data) override;
     bool parseMessageChat(network::Packet& packet, MessageChatData& data) override;
     bool parseGameObjectQueryResponse(network::Packet& packet, GameObjectQueryResponseData& data) override;

src/game/game_handler.cpp

@@ -7626,7 +7626,9 @@ void GameHandler::useItemBySlot(int backpackIndex) {
     }
     if (itemGuid != 0 && state == WorldState::IN_WORLD && socket) {
         // WoW inventory: equipment 0-18, bags 19-22, backpack 23-38
-        auto packet = UseItemPacket::build(0xFF, static_cast<uint8_t>(23 + backpackIndex), itemGuid);
+        auto packet = packetParsers_
+            ? packetParsers_->buildUseItem(0xFF, static_cast<uint8_t>(23 + backpackIndex), itemGuid)
+            : UseItemPacket::build(0xFF, static_cast<uint8_t>(23 + backpackIndex), itemGuid);
         socket->send(packet);
     } else if (itemGuid == 0) {
         LOG_WARNING("Use item failed: missing item GUID for slot ", backpackIndex);

src/game/packet_parsers_classic.cpp

@@ -260,16 +260,16 @@ network::Packet ClassicPacketParsers::buildMovementPacket(LogicalOpcode opcode,
 // ============================================================================
 // Classic buildCastSpell
 // Vanilla 1.12.x: NO castCount prefix, NO castFlags byte
-// Format: uint32 spellId + uint32 targetFlags + [PackedGuid if unit target]
+// Format: uint32 spellId + uint16 targetFlags + [PackedGuid if unit target]
 // ============================================================================
 network::Packet ClassicPacketParsers::buildCastSpell(uint32_t spellId, uint64_t targetGuid, uint8_t /*castCount*/) {
     network::Packet packet(wireOpcode(LogicalOpcode::CMSG_CAST_SPELL));
 
     packet.writeUInt32(spellId);
 
-    // SpellCastTargets — vanilla/CMaNGOS uses uint32 target mask (same as WotLK)
+    // SpellCastTargets — vanilla/CMaNGOS uses uint16 target mask (WotLK uses uint32)
     if (targetGuid != 0) {
-        packet.writeUInt32(0x02); // TARGET_FLAG_UNIT
+        packet.writeUInt16(0x02); // TARGET_FLAG_UNIT
 
         // Write packed GUID
         uint8_t mask = 0;
@@ -289,12 +289,26 @@ network::Packet ClassicPacketParsers::buildCastSpell(uint32_t spellId, uint64_t
             packet.writeUInt8(bytes[i]);
         }
     } else {
-        packet.writeUInt32(0x00); // TARGET_FLAG_SELF
+        packet.writeUInt16(0x00); // TARGET_FLAG_SELF
     }
 
     return packet;
 }
 
+// ============================================================================
+// Classic CMSG_USE_ITEM
+// Vanilla 1.12.x: bag(u8) + slot(u8) + spellIndex(u8) + SpellCastTargets(u16)
+// NO spellId, itemGuid, glyphIndex, or castFlags fields (those are WotLK)
+// ============================================================================
+network::Packet ClassicPacketParsers::buildUseItem(uint8_t bagIndex, uint8_t slotIndex, uint64_t /*itemGuid*/) {
+    network::Packet packet(wireOpcode(LogicalOpcode::CMSG_USE_ITEM));
+    packet.writeUInt8(bagIndex);
+    packet.writeUInt8(slotIndex);
+    packet.writeUInt8(0);       // spell_index (which item spell to trigger, usually 0)
+    packet.writeUInt16(0x0000); // SpellCastTargets: TARGET_FLAG_SELF
+    return packet;
+}
+
 // ============================================================================
 // Classic SMSG_CAST_FAILED: no castCount byte (added in TBC/WotLK)
 // Format: spellId(u32) + result(u8)