game: add Classic 1.12 overrides for parseSpellStart and parseSpellGo

Vanilla 1.12 SMSG_SPELL_START and SMSG_SPELL_GO use:
- PackedGuid (variable-length) for caster and target GUIDs, not full uint64
- uint16 castFlags, not uint32 as in TBC/WotLK
- uint16 targetFlags in SpellCastTargets, not uint32

Without these overrides Classic inherited TBC's implementations which
read 8 bytes for each GUID (over-reading the PackedGuid) and then 4
bytes for castFlags instead of 2, misaligning all subsequent fields
and producing garbage spell IDs, cast times, and target GUIDs.

Hit and miss target GUIDs in SMSG_SPELL_GO are also PackedGuid in
Vanilla (vs full uint64 in TBC), handled by the new parseSpellGo.

include/game/packet_parsers.hpp

@@ -405,6 +405,9 @@ public:
     bool parseMonsterMove(network::Packet& packet, MonsterMoveData& data) override {
         return MonsterMoveParser::parseVanilla(packet, data);
     }
+    // Classic 1.12 uses PackedGuid (not full uint64) and uint16 castFlags (not uint32)
+    bool parseSpellStart(network::Packet& packet, SpellStartData& data) override;
+    bool parseSpellGo(network::Packet& packet, SpellGoData& data) override;
 };
 
 /**