feat(game): introduce GameHandler domain interfaces and eliminate friend declarations

Add game_interfaces.hpp with five narrow domain contracts that GameHandler now
publishes to its domain handlers, replacing the previous friend-class anti-pattern.

Changes:
- include/game/game_interfaces.hpp (new): IConnectionState, ITargetingState,
  IEntityAccess, ISocialState, IPvpState — each interface exposes only the state
  its consumer legitimately needs
- include/game/game_handler.hpp: GameHandler inherits all five interfaces;
  include of game_interfaces.hpp added
- include/game/movement_handler.hpp: remove `friend class GameHandler`; add
  public named accessors for previously-private fields (monsterMovePacketsThisTickRef,
  timeSinceLastMoveHeartbeatRef, resetMovementClock, setFalling, setFallStartMs)
- include/game/spell_handler.hpp: remove `friend class GameHandler/InventoryHandler/
  CombatHandler/EntityController`; promote private packet handlers (handlePetSpells,
  handleListStabledPets, pet stable commands, DBC loaders) to public; add accessor
  methods for aura cache, known spells, and player aura slot mutation
- src/game/game_handler.cpp, game_handler_callbacks.cpp, game_handler_packets.cpp:
  replace direct private field access with the new accessor API
  (e.g. casting_ → isCasting(), monsterMovePacketsThisTick_ → ...ThisTickRef())
- src/game/inventory_handler.cpp, combat_handler.cpp, entity_controller.cpp:
  replace friend-class private access with public accessor calls

No behaviour change. All 13 test suites pass. Zero build warnings.

src/game/combat_handler.cpp

@@ -1065,7 +1065,7 @@ void CombatHandler::setTarget(uint64_t guid) {
 
     // Clear stale aura data from the previous target so the buff bar shows
     // an empty state until the server sends SMSG_AURA_UPDATE_ALL for the new target.
-    if (owner_.getSpellHandler()) for (auto& slot : owner_.getSpellHandler()->targetAuras_) slot = AuraSlot{};
+    if (owner_.getSpellHandler()) owner_.getSpellHandler()->clearTargetAuras();
 
     // Clear previous target's cast bar on target change
     // (the new target's cast state is naturally fetched from spellHandler_->unitCastStates_ by GUID)