feat: Add multiplayer networking architecture and documentation updates

- Updated Master Blueprint Index to include Multiplayer Networking support.
- Added detailed Multiplayer Networking sections across all developer documentation (01-11).
- Introduced authority maps, key patterns, and RPC guidelines for each system.
- Established a comprehensive multiplayer networking architecture document outlining core principles, replication strategies, and anti-cheat considerations.
- Enhanced UI documentation to clarify local-only behavior and binding to replicated dispatchers.
- Implemented client prediction strategies and RPC naming conventions for consistency across the framework.

docs/developer/04-inventory-systems.md

@@ -240,4 +240,29 @@
 
 ---
 
+## Multiplayer Networking
+
+### Category Authority Map
+| System | Authority | Anti-Cheat |
+|--------|-----------|------------|
+| `BPC_InventorySystem` | Server-authoritative add/remove/use/drop | Server validates slot space, weight, stack limits |
+| `BPC_ContainerInventory` | Server-authoritative open/loot/transfer | Server validates distance, lock, slot availability |
+| `BP_ItemPickup` | Server-authoritative pickup | Server validates inventory space before removing pickup |
+| `BPC_EquipmentSlotSystem` | Server-authoritative equip/unequip | Server validates slot type compatibility |
+| `BPC_ConsumableSystem` | Server-authoritative use | Server validates item exists, cooldown, applies effect |
+| `BPC_ItemCombineSystem` | Server-authoritative combine | Server validates recipe from DA_ItemData |
+
+### Server RPCs
+- `Server_AddItem`, `Server_RemoveItem`, `Server_UseItem`, `Server_DropItem`, `Server_TransferItem`
+- `Server_EquipItem`, `Server_UnequipItem`
+- `Server_LootContainer`, `Server_PickupItem`
+
+### Inventory Replication Strategy
+- **Slots array** replicated with `OnRep_Slots` → recalculates weight, fires `OnInventoryChanged`
+- **Client predicts UI** (item moves visually on drag-drop); server validates and corrects via OnRep
+- **Server validates all transactions** against slot space, weight capacity, stack limits, and category restrictions
+- **Cooldowns** managed locally (not replicated) but validated on server for authoritative actions
+
+---
+
 *Developer Reference v1.0 — 04 Inventory Systems. Companion to docs/blueprints/04-inventory/ specs.*