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/06-ui-systems.md

@@ -154,4 +154,23 @@ Binds to `BPC_InteractionDetector.OnTargetFound/Lost` and `OnHoldProgressUpdated
 
 ---
 
+## Multiplayer Networking
+
+**All UI is local per-client. Zero networking code in widgets.**
+
+### UI Networking Pattern
+```
+Server state changes → replicated variable → OnRep fires dispatcher
+  → Widget (local) already bound to that dispatcher
+  → Widget updates with zero multiplayer awareness
+```
+
+### Category Rules
+- **WBP_* Widgets:** Local only. Bind to replicated dispatchers from gameplay systems. Use the same binding code as single-player.
+- **SS_UIManager:** Local per-client. Menu stack is per-player — different players can be in different menus.
+- **Input mode changes:** Handled per-client by `SS_EnhancedInputManager` listening to game phase dispatchers.
+- **HUD visibility:** Each client independently determines what to show based on their replicated state (e.g., hiding player sees different HUD than exposed player).
+
+---
+
 *Developer Reference v1.0 — 06 UI Systems. Companion to docs/blueprints/06-ui/ specs.*