shared-tun: preserve member ownership on mobile

Introduce shared NIC source ownership and dispatcher handling so a
single dev_name can be shared by multiple tun-enabled instances while
keeping per-member IP and route claims distinct.

Pass Android VpnService fd registration with per-instance source and
route claims. Keep the VPN address list limited to real member
addresses and allow AF_INET6 without installing hidden fd00::1.

Invalidate dispatcher flow and NAT state when source ownership changes
or a member unregisters. Avoid rewriting non-first IPv4 fragment
payloads, and adjust fragmented TCP/UDP checksums without recomputing
over partial fragment bodies.

Preserve source-owner routing for equal-prefix route conflicts, keep
ICMP echo NAT entries distinct by echo id, and retry stale flow-owner
send failures from the original packet. Only record NAT state after a
translated packet is accepted by its member.

Apply Linux IPv4 route preferred-source hints for shared routes and keep
route repair paths source-aware. Keep Darwin ifcfg access scoped to
cleanup-only paths where netns is not available.

tauri-plugin-vpnservice/android/src/main/java/TauriVpnService.kt

@@ -5,6 +5,7 @@ import android.net.VpnService
 import android.os.Build
 import android.os.ParcelFileDescriptor
 import android.os.Bundle
+import android.system.OsConstants.AF_INET6
 import java.net.InetAddress
 import java.util.Arrays
 
@@ -115,7 +116,7 @@ class TauriVpnService : VpnService() {
             if (ipParts.size != 2) throw IllegalArgumentException("Invalid IP addr string")
             builder.addAddress(ipParts[0], ipParts[1].toInt())
         }
-        builder.addAddress("fd00::1", 128)
+        builder.allowFamily(AF_INET6)
 
         builder.setMtu(mtu)
         dns?.let { builder.addDnsServer(it) }