Add router-on-a-stick, commit-confirm rollback and an example config

- Allow the WAN link to also carry LANs when the uplink is PPPoE on a
  VLAN with IPv6 off; only the PPP device counts as an uplink then, so
  LAN traffic can never match WAN rules.
- Add router.rollback: router-arm / router-confirm / router-rollback
  plus `nix run .#deploy`, a commit-confirm deployment flow.
- Wait for the LAN devices instead of the uplink in networkd
  wait-online, so an unplugged uplink cannot block boot or activation.
- Add examples/desktop (single NIC, PPPoE on VLAN 1001) as
  nixosConfigurations.example, built as a flake check.

Covered by the onestick and rollback VM tests and new assertion cases.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

modules/router/networking.nix

@@ -5,11 +5,15 @@ let
   pppoe = cfg.wan.ipv4.type == "pppoe";
   vlan = cfg.wan.ipv4.pppoe.vlan;
   vlanDevice = "${cfg.wan.interface}.${toString vlan}";
+  wanVlanOn = parent: lib.optional (pppoe && vlan != null && parent == cfg.wan.interface) vlanDevice;
 
   lans = lib.attrValues cfg.lans;
   vlanLans = lib.filter (l: l.vlan != null) lans;
   # VLAN devices hang off their parent; the parent's own unit must list them.
-  vlansOn = parent: map (l: l.device) (lib.filter (l: l.interface == parent) vlanLans);
+  vlansOn =
+    parent: map (l: l.device) (lib.filter (l: l.interface == parent) vlanLans) ++ wanVlanOn parent;
+  # The WAN link's own unit is only needed when no LAN shares the link.
+  sharedLink = lib.elem cfg.wan.interface (map (l: l.interface) lans);
   untaggedParents = map (l: l.interface) (lib.filter (l: l.vlan == null) lans);
   # Parents that carry only tagged LANs still need a unit to be brought up.
   trunkParents = lib.filter (p: !(lib.elem p untaggedParents)) (
@@ -22,6 +26,12 @@ in
   config = lib.mkIf cfg.enable {
     networking.useNetworkd = true;
     networking.useDHCP = false;
+    # Wait for the LAN devices (VLANs included: Kea only binds interfaces that
+    # exist when it starts) but not for the uplink: an ISP outage or a pulled
+    # cable must not hold up boot or a deployment.
+    systemd.network.wait-online.extraArgs = map (l: "--interface=${l.device}") (
+      lib.attrValues cfg.lans
+    );
 
     boot.kernel.sysctl = {
       "net.ipv4.conf.all.forwarding" = 1;
@@ -49,87 +59,88 @@ in
         }
       ) (lib.filterAttrs (_: l: l.vlan != null) cfg.lans);
 
-    systemd.network.networks = {
-      "10-wan" = {
-        matchConfig.Name = cfg.wan.interface;
-        networkConfig = {
-          DHCP = dhcp;
+    systemd.network.networks =
+      lib.optionalAttrs (!sharedLink) {
+        "10-wan" = {
+          matchConfig.Name = cfg.wan.interface;
+          networkConfig = {
+            DHCP = dhcp;
+          }
+          // lib.optionalAttrs v6 {
+            IPv6AcceptRA = true;
+            KeepConfiguration = "yes";
+          }
+          // lib.optionalAttrs (pppoe && vlan != null) {
+            VLAN = [ vlanDevice ];
+          };
+        }
+        // lib.optionalAttrs (!pppoe) {
+          # The router runs its own resolver; never adopt the ISP's DNS for the host.
+          dhcpV4Config.UseDNS = false;
         }
         // lib.optionalAttrs v6 {
-          IPv6AcceptRA = true;
-          KeepConfiguration = "yes";
-        }
-        // lib.optionalAttrs (pppoe && vlan != null) {
-          VLAN = [ vlanDevice ];
+          # The ISP sends RAs (default route) without DHCPv6 flags, so solicit anyway.
+          dhcpV6Config = {
+            WithoutRA = "solicit";
+            UseDelegatedPrefix = true;
+            UseDNS = false;
+          };
         };
       }
-      // lib.optionalAttrs (!pppoe) {
-        # The router runs its own resolver; never adopt the ISP's DNS for the host.
-        dhcpV4Config.UseDNS = false;
-      }
-      // lib.optionalAttrs v6 {
-        # The ISP sends RAs (default route) without DHCPv6 flags, so solicit anyway.
-        dhcpV6Config = {
-          WithoutRA = "solicit";
-          UseDelegatedPrefix = true;
-          UseDNS = false;
-        };
-      };
-    }
-    // lib.optionalAttrs (pppoe && vlan != null) {
-      # pppd configures the session; networkd only has to bring the device up.
-      "11-wan-pppoe" = {
-        matchConfig.Name = vlanDevice;
-        networkConfig = {
-          LinkLocalAddressing = "no";
-          IPv6AcceptRA = false;
-        };
-      };
-    }
-    // lib.mapAttrs' (
-      name: lan:
-      lib.nameValuePair "20-lan-${name}" (
-        {
-          matchConfig.Name = lan.device;
-          address = [ "${lan.ipv4.address}/${toString lan.ipv4.prefixLength}" ];
-          networkConfig = {
-            ConfigureWithoutCarrier = true;
-            # Never accept RAs from the inside.
-            IPv6AcceptRA = false;
-          }
-          // lib.optionalAttrs (lan.vlan == null && vlansOn lan.interface != [ ]) {
-            VLAN = vlansOn lan.interface;
-          }
-          // lib.optionalAttrs lan.ipv6.pd {
-            DHCPPrefixDelegation = true;
-            IPv6SendRA = true;
-          };
-        }
-        // lib.optionalAttrs lan.ipv6.pd {
-          dhcpPrefixDelegationConfig = {
-            UplinkInterface = cfg.wan.interface;
-            SubnetId = 0;
-            Announce = true;
-            Assign = true;
-          };
-          # No RDNSS yet: the global address is unknown at evaluation time.
-          ipv6SendRAConfig.EmitDNS = false;
-        }
-      )
-    ) cfg.lans
-    // lib.listToAttrs (
-      map (
-        parent:
-        lib.nameValuePair "19-trunk-${parent}" {
-          matchConfig.Name = parent;
+      // lib.optionalAttrs (pppoe && vlan != null) {
+        # pppd configures the session; networkd only has to bring the device up.
+        "11-wan-pppoe" = {
+          matchConfig.Name = vlanDevice;
           networkConfig = {
             LinkLocalAddressing = "no";
             IPv6AcceptRA = false;
-            ConfigureWithoutCarrier = true;
-            VLAN = vlansOn parent;
           };
-        }
-      ) trunkParents
-    );
+        };
+      }
+      // lib.mapAttrs' (
+        name: lan:
+        lib.nameValuePair "20-lan-${name}" (
+          {
+            matchConfig.Name = lan.device;
+            address = [ "${lan.ipv4.address}/${toString lan.ipv4.prefixLength}" ];
+            networkConfig = {
+              ConfigureWithoutCarrier = true;
+              # Never accept RAs from the inside.
+              IPv6AcceptRA = false;
+            }
+            // lib.optionalAttrs (lan.vlan == null && vlansOn lan.interface != [ ]) {
+              VLAN = vlansOn lan.interface;
+            }
+            // lib.optionalAttrs lan.ipv6.pd {
+              DHCPPrefixDelegation = true;
+              IPv6SendRA = true;
+            };
+          }
+          // lib.optionalAttrs lan.ipv6.pd {
+            dhcpPrefixDelegationConfig = {
+              UplinkInterface = cfg.wan.interface;
+              SubnetId = 0;
+              Announce = true;
+              Assign = true;
+            };
+            # No RDNSS yet: the global address is unknown at evaluation time.
+            ipv6SendRAConfig.EmitDNS = false;
+          }
+        )
+      ) cfg.lans
+      // lib.listToAttrs (
+        map (
+          parent:
+          lib.nameValuePair "19-trunk-${parent}" {
+            matchConfig.Name = parent;
+            networkConfig = {
+              LinkLocalAddressing = "no";
+              IPv6AcceptRA = false;
+              ConfigureWithoutCarrier = true;
+              VLAN = vlansOn parent;
+            };
+          }
+        ) trunkParents
+      );
   };
 }