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/blueprints/02-player/08_BPC_HealthSystem.md

@@ -337,4 +337,70 @@ flowchart TD
 
 ---
 
+## 11. Multiplayer Networking (Expanded)
+
+### Replicated Variables (Existing + New)
+
+| Variable | Type | Condition | Description |
+|----------|------|-----------|-------------|
+| `CurrentHealth` | `Float` | `Replicated Using OnRep_CurrentHealth` | Clients sync via OnRep → dispatcher |
+| `DeathState` | `E_DeathState` | `Replicated Using OnRep_DeathState` | Death state synced; OnRep triggers death effects |
+| `bIsInvincible` | `Boolean` | `Replicated` | Invincibility visible to other players |
+| `MaxHealth` | `Float` | `Replicated` | Synced for UI percentage calculations |
+
+### Server RPCs
+
+| RPC | Direction | Description |
+|-----|-----------|-------------|
+| `Server_ApplyDamage` | Client→Server | Client reports taking damage. Server validates source, range, damage type. |
+| `Server_ApplyHealing` | Client→Server | Client requests healing. Server validates heal source, applies. |
+| `Server_KillInstant` | Client→Server | Only accepted from authoritative sources (GM, traps). Client calls are rejected. |
+
+### Authority Gates
+
+```
+Function ApplyDamage(DamageEvent)
+  Switch HasAuthority
+    Authority:
+      → Calculate resistance, apply damage, fire dispatchers
+    Remote:
+      → Return (clients never modify health directly)
+      → Server_ApplyDamage is called by damage source (weapon/trap), not victim
+
+Function ApplyHealing(HealAmount)
+  Switch HasAuthority
+    Authority:
+      → Apply healing, fire dispatchers
+    Remote:
+      → Return
+      → Consumable use calls Server_UseConsumable → server calls ApplyHealing
+```
+
+### Client Prediction
+
+- **Health bar:** Client predicts damage flash on hit; server-corrected value arrives via `OnRep_CurrentHealth`.
+- **Death:** No prediction. `OnRep_DeathState` triggers all death effects (screen fade, ragdoll).
+- **Healing:** Client shows green flash immediately + predicted health; server corrects within one RTT.
+- **Invincibility:** Visual effect (shield shimmer) is local cosmetic; state is replicated.
+
+### OnRep Handlers
+
+```
+OnRep_CurrentHealth()
+  → Broadcast OnHealthChanged(OldHealth, CurrentHealth, Delta)
+  → UI updates identically to single-player path
+
+OnRep_DeathState()
+  → Broadcast OnDeathStateChanged(OldState, NewState)
+  → If Dead: play death animation, screen effects, notify GM
+```
+
+### Anti-Cheat
+
+- Server validates all `Server_ApplyDamage` calls: check instigator range, weapon fire rate, damage type validity.
+- Server never trusts `DamageEvent.BaseAmount` from client — always recalculates from weapon data.
+- `Server_KillInstant` is only callable from server-authoritative systems (traps, death zones, GM).
+
+---
+
 *Blueprint Spec: Health System. Conforms to TEMPLATE.md v1.0 — part of the UE5 Modular Game Framework.*