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.

Use smoltcp for packet parsing and checksum updates. Keep core CI
matrix jobs independent.

Reserve native inbound flows before source translations allocate their
wire tuples. Keep active reservations under table pressure and leave
forwarded subnet traffic outside the local flow table.

Complete interface claim updates on the shared runtime and retain member
registrations until the transaction commits. Keep address, route, and
MTU cleanup ordered after cancelled callers, including device
initialization.

Project mobile VPN addresses, routes, and DNS from admitted native
members. Prefer unchanged attached claims and report conflicting members
individually. Apply each member's complete initial claims before opening
the shared fd, so conflicts leave healthy members usable.

Cover inbound IPv4/IPv6 collisions, cancellation during native changes
and dispatcher acknowledgments, admission priority, and atomic mobile
claim rejection with regression tests.

Use scoped TestNetwork fixtures for shared TUN bridge setup and cleanup.
Resolve public IPv6 namespace aliases through the lab's actual names so
the new tests remain isolated under parallel upstream test runs.

.github/workflows/core.yml

@@ -144,7 +144,7 @@ jobs:
             easytier-web/frontend/dist/*
   build:
     strategy:
-      fail-fast: true
+      fail-fast: false
       matrix:
         include:
           - TARGET: x86_64-unknown-linux-musl