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/network/world_socket.cpp

@@ -15,6 +15,9 @@
 
 namespace {
 constexpr size_t kMaxReceiveBufferBytes = 8 * 1024 * 1024;
+// Per-frame packet budgets prevent a burst of server data from starving the
+// render loop. Tunable via env vars for debugging heavy-traffic scenarios
+// (e.g. SMSG_UPDATE_OBJECT floods on login to crowded zones).
 constexpr int kDefaultMaxParsedPacketsPerUpdate = 64;
 constexpr int kAbsoluteMaxParsedPacketsPerUpdate = 220;
 constexpr int kMinParsedPacketsPerUpdate = 8;