docs(report): add section 5.3 draft assets

docs/temp-section-5-3-diagrams/5.3.3-device-registration-and-presence.md

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+# 5.3.3 Device Identity Registration and Presence
+
+This diagram shows how a signed-in user registers a browser as a device and how presence is maintained after realtime connection.
+
+```mermaid
+sequenceDiagram
+    autonumber
+    participant User as Signed-in User
+    participant Web as WebApp Controller
+    participant Devices as POST /devices/register
+    participant DB as devices + device_links
+    participant Token as Device Token
+    participant Socket as Socket.IO Gateway
+
+    User->>Web: submit onboarding role + name
+    Web->>Devices: register device
+    Devices->>DB: insert devices row
+    Devices->>DB: auto-link to opposite-role devices if present
+    Devices-->>Web: return device + deviceToken
+    Web->>Web: persist saved device record
+    Web->>Socket: connect with device token
+    Socket->>DB: mark device online
+    Web->>Socket: periodic heartbeat
+    Socket->>DB: update lastSeenAt and status
+    Socket-->>Web: connection + heartbeat acknowledgements
+```