Implement SMSG_RESUME_CAST_BAR, SMSG_THREAT_UPDATE, SMSG_UPDATE_INSTANCE_ENCOUNTER_UNIT

- SMSG_RESUME_CAST_BAR: parse packed_guid caster/target + spellId + remainingMs +
  totalMs; restores cast bar state when server re-syncs a cast in progress
- SMSG_THREAT_UPDATE: properly consume packed_guid host/target + threat entries
  to suppress unhandled packet warnings
- SMSG_UPDATE_INSTANCE_ENCOUNTER_UNIT: track up to 5 boss encounter unit guids
  per slot; expose via getEncounterUnitGuid(slot); clear on world transfer
  These guids identify active boss units for raid/boss frame display.

include/game/game_handler.hpp

@@ -772,6 +772,10 @@ public:
         bool     extended    = false;
     };
     const std::vector<InstanceLockout>& getInstanceLockouts() const { return instanceLockouts_; }
+    // Returns boss unit guid for the given encounter slot (0 if none)
+    uint64_t getEncounterUnitGuid(uint32_t slot) const {
+        return (slot < kMaxEncounterSlots) ? encounterUnitGuids_[slot] : 0;
+    }
 
     // ---- LFG / Dungeon Finder ----
     enum class LfgState : uint8_t {
@@ -1738,6 +1742,10 @@ private:
     // Instance / raid lockouts
     std::vector<InstanceLockout> instanceLockouts_;
 
+    // Instance encounter boss units (slots 0-4 from SMSG_UPDATE_INSTANCE_ENCOUNTER_UNIT)
+    static constexpr uint32_t kMaxEncounterSlots = 5;
+    std::array<uint64_t, kMaxEncounterSlots> encounterUnitGuids_ = {};  // 0 = empty slot
+
     // LFG / Dungeon Finder state
     LfgState lfgState_        = LfgState::None;
     uint32_t lfgDungeonId_    = 0;   // current dungeon entry