feat: add RAID_TARGET_UPDATE event and GetRaidTargetIndex/SetRaidTarget

Fire RAID_TARGET_UPDATE event when raid markers (skull, cross, etc.)
are set or cleared on targets. Add two Lua API functions:
- GetRaidTargetIndex(unit) returns marker index 1-8 (or nil)
- SetRaidTarget(unit, index) sets marker 1-8 (or 0 to clear)

Enables raid marking addons and nameplate addons that display raid
icons to react to marker changes in real-time.

src/game/game_handler.cpp

@@ -5022,6 +5022,8 @@ void GameHandler::handlePacket(network::Packet& packet) {
                 }
             }
             LOG_DEBUG("MSG_RAID_TARGET_UPDATE: type=", static_cast<int>(rtuType));
+            if (addonEventCallback_)
+                addonEventCallback_("RAID_TARGET_UPDATE", {});
             break;
         }
         case Opcode::SMSG_BUY_ITEM: {