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/01-core-foundation.md

@@ -377,4 +377,28 @@ MainMenu → Loading → InGame ⇄ Paused
 
 ---
 
+## Multiplayer Networking
+
+### Category Authority Map
+| System | Role | Authority |
+|--------|------|-----------|
+| `GI_GameFramework` | GamePhase, session flags | Server sets; clients read via replicated dispatcher |
+| `GM_CoreGameMode` | Player spawning, chapter transitions, death routing | Server-only (extends replicated GameMode) |
+| `GS_CoreGameState` | Shared session state bus | **Fully replicated** — 5 vars with OnRep handlers |
+| `GI_GameTagRegistry` | Tag documentation | Read-only config — identical on all clients |
+| `FL_GameUtilities` | Static utilities | No state — callable from any side |
+| `I_InterfaceLibrary` | Communication contracts | Interface calls work on any side; mutators must check authority |
+| `DA_ItemData` | Item definitions | Read-only Data Asset — identical on all clients |
+
+### Key Patterns
+- **GS_CoreGameState as Replication Hub:** All shared session state (chapter, phase, objectives, encounter status) writes through GS_CoreGameState which auto-replicates to all clients via OnRep → dispatcher.
+- **GamePhase transitions are server-authoritative:** `SetGamePhase()` checks `HasAuthority()`. Clients receive phase changes via `OnRep_GamePhase` → `OnGamePhaseChanged` dispatcher.
+- **Interface calls route through server:** `I_Interactable.Execute_OnInteract` must be called server-side. Clients call `Server_Interact` RPC → server validates → calls interface.
+- **Data Assets are client-ready:** All `DA_*` assets load identically on all instances from disk. No replication needed.
+
+### Full Spec
+See [`docs/architecture/multiplayer-networking.md`](../architecture/multiplayer-networking.md) Section 3.1.
+
+---
+
 *Developer Reference v1.0 — 01 Core Foundation Systems. Companion to docs/blueprints/01-core/ specs.*