refactor: replace magic bag slot offset 19 with FIRST_BAG_EQUIP_SLOT

- Add Inventory::FIRST_BAG_EQUIP_SLOT = 19 constant with why-comment
  explaining WoW equip slot layout (bags occupy slots 19-22)
- Replace all 19 occurrences of magic number 19 in bag slot calculations
  across inventory_handler, spell_handler, inventory, and game_handler
- Add UNIT_FIELD_FLAGS / UNIT_FLAG_PVP comment in combat_handler
- Add why-comment on network packet budget constants (prevent server
  data bursts from starving the render loop)

src/game/combat_handler.cpp

@@ -1362,6 +1362,7 @@ void CombatHandler::togglePvp() {
     auto entity = owner_.getEntityManager().getEntity(owner_.playerGuid);
     bool currentlyPvp = false;
     if (entity) {
+        // UNIT_FIELD_FLAGS (index 59), bit 0x1000 = UNIT_FLAG_PVP
         currentlyPvp = (entity->getField(59) & 0x00001000) != 0;
     }
     if (currentlyPvp) {