Add tech_docs/lab/ipv6_lab.md

tech_docs/lab/ipv6_lab.md

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+```mermaid
+graph TD
+  subgraph Host Machine (Linux + LXD)
+    H0[LXD Host]
+    H0 --> B1[br-lan<br>IPv4/IPv6 Dual-Stack LAN Bridge]
+    H0 --> B2[br-wan<br>Simulated Internet]
+  end
+
+  subgraph LAN Zone
+    B1 --> R1[Router<br>VyOS or Ubuntu w/ radvd, DHCPv6, NAT64]
+    R1 --> C1[Client1<br>IPv6-only]
+    R1 --> C2[Client2<br>Dual-Stack]
+    R1 --> C3[Client3<br>IPv4-only]
+    R1 --> S1[WebServer<br>Dual-Stack<br>Nginx + DNS]
+  end
+
+  subgraph WAN Zone (Simulated Internet)
+    B2 --> ISP[IPv6+IPv4 Simulation Router<br>(can be simple Ubuntu router)]
+    ISP --> EXT[External IPv6 Service<br>(e.g., public web or tunnelbroker)]
+  end
+
+  %% Connections
+  R1 -->|Default route| ISP
+  C1 -->|NAT64| R1
+  C3 -->|IPv4 DHCP| R1
+  C2 -->|SLAAC + DHCPv6| R1
+```