Add Tier 7 commands: combat and trade

Combat Commands:
- /duel - Challenge target to a duel (CMSG_DUEL_PROPOSED 0x166)
- /trade - Open trade window with target (CMSG_INITIATE_TRADE 0x116)
- /startattack - Begin auto-attacking target
- /stopattack - Stop auto-attacking
- /stopcasting - Cancel current spell cast

New opcodes:
- CMSG_DUEL_PROPOSED (0x166) for initiating duels
- CMSG_INITIATE_TRADE (0x116) for starting trades

Packet builders:
- DuelProposedPacket - sends duel challenge to target GUID
- InitiateTradePacket - sends trade request to target GUID
- AttackSwingPacket, AttackStopPacket, CancelCastPacket reused from existing

Game handler methods:
- proposeDuel(targetGuid) - challenge target to duel
- initiateTrade(targetGuid) - open trade with target
- stopCasting() - cancel current spell cast (uses existing casting state)

All commands include validation for target selection and world state.
Removed duplicate packet class definitions from previous phases.

include/game/opcodes.hpp

@@ -106,10 +106,14 @@ enum class Opcode : uint16_t {
     MSG_RAID_READY_CHECK_CONFIRM     = 0x3AE,
 
     // ---- Duel ----
+    CMSG_DUEL_PROPOSED               = 0x166,
     CMSG_DUEL_ACCEPTED               = 0x16C,
     CMSG_DUEL_CANCELLED              = 0x16D,
     SMSG_DUEL_REQUESTED              = 0x167,
 
+    // ---- Trade ----
+    CMSG_INITIATE_TRADE              = 0x116,
+
     // ---- Random Roll ----
     MSG_RANDOM_ROLL                  = 0x1FB,