Add shared virtual NIC core with per-member routing

Share a named TUN across instances while keeping unnamed devices
dedicated. Track member addresses, routes, and MTU, and reject
overlapping destinations except the common Magic DNS host route.

Dispatch packets between the physical TUN and member tunnels. Keep
IPv4 and ordinary IPv6 source translation for mobile wrong-source
traffic. Update platform configuration and runtime host integration,
with unit and network namespace coverage for routing and lifecycle.

Add the packet dependency and keep core CI matrix jobs independent.

.github/workflows/core.yml

@@ -82,7 +82,7 @@ jobs:
             easytier-web/frontend/dist/*
   build:
     strategy:
-      fail-fast: true
+      fail-fast: false
       matrix:
         include:
           - TARGET: x86_64-unknown-linux-musl