machines/stellio: stop deprecating a PD prefix the ISP re-delegates (sync with cnx-network 45643d6)

The ISP modem restart at 08:19 re-delegated the same /64 five seconds
after the PPPoE drop. networkd re-announced it at once, but the fixed
7 s radvd run kept sending preferred-lifetime-0 RAs for it, so clients
sat on a deprecated prefix until networkd's next periodic RA and IPv6
stayed broken for minutes.

Send one RA per radvd start, up to three 3 s apart, and skip each one if
the /64 is already back on the interface. Verified on stellio with the
rendered script: a prefix gone for good -> three RAs; the PD address
removed and re-added 4 s later -> two RAs, then "is back, leaving it
announced", client preferred again and pinging over v6.

machines/stellio/configuration.nix

@@ -108,7 +108,7 @@
     systemd.network.networks."40-vlan-lan".dhcpPrefixDelegationConfig.SubnetId = lib.mkForce "0x0";
     systemd.network.networks."40-vlan-mgmt".dhcpPrefixDelegationConfig.SubnetId = lib.mkForce "0x1";
 
-    # Interim copy of cnx-network-clan fa11327 (router/ipv6: deprecate
+    # Interim copy of cnx-network-clan 45643d6 (router/ipv6: deprecate
     # withdrawn DHCPv6-PD prefixes); drop when the input is bumped past it.
     # networkd never deprecates a prefix it stops announcing (systemd#29651):
     # on every reboot the first PPPoE session lands on another AIS BNG, is
@@ -117,10 +117,12 @@
     # one preferred for the rest of its 1800 s lifetime, so RFC 6724 source
     # selection picks the dead prefix and every v6 connection fails for up
     # to 30 minutes (2026-10-08 07:57). Watch vlan-* for networkd removing a
-    # dynamic (PD) address and send a few RAs with that /64 at preferred
-    # lifetime 0; radvd is only the packet generator, killed hard after ~7 s
-    # so it sends no shutdown RA (router lifetime 0) and does not keep
-    # deprecating a prefix the ISP re-delegates on reconnect.
+    # dynamic (PD) address and send up to three RAs with that /64 at
+    # preferred lifetime 0, one short-lived radvd per RA (SIGKILLed: no
+    # shutdown RA with router lifetime 0), stopping as soon as the prefix is
+    # back on the interface -- a plain flap re-delegates the same /64 within
+    # seconds and a deprecating RA sent after networkd re-announced it would
+    # leave clients deprecated until the next periodic RA (08:19 today).
     systemd.services.router-radv-deprecate = {
       description = "Deprecate withdrawn DHCPv6-PD prefixes on the VLANs";
       wantedBy = [ "multi-user.target" ];
@@ -138,11 +140,18 @@
       script = ''
         conf=$RUNTIME_DIRECTORY/radvd.conf
         pid=$RUNTIME_DIRECTORY/radvd.pid
+
+        # /64s of the global addresses currently on an interface.
+        prefixes_on() {
+          ip -o -6 addr show dev "$1" scope global | while read -r _ _ _ a _; do
+            echo "$(ipcalc --no-decorate -n "$a")/''${a##*/}"
+          done
+        }
+
         # "Deleted 4: vlan-lan    inet6 2001:db8::1/64 scope global dynamic mngtmpaddr \ valid_lft ..."
         ip -o -6 monitor address | while read -r event _ dev _ addr _ scope rest; do
           [[ $event == Deleted && $dev == vlan-* && $scope == global && $rest == *dynamic* ]] || continue
           prefix="$(ipcalc --no-decorate -n "$addr")/''${addr##*/}"
-          echo "$dev: deprecating withdrawn prefix $prefix"
           cat > "$conf" <<EOF
         interface $dev {
           AdvSendAdvert on;
@@ -158,11 +167,20 @@
           };
         };
         EOF
+          for _ in 1 2 3; do
+            if prefixes_on "$dev" | grep -qxF "$prefix"; then
+              echo "$dev: $prefix is back, leaving it announced"
+              break
+            fi
+            echo "$dev: deprecating withdrawn prefix $prefix"
             rm -f "$pid"
+            # radvd sends its first RA on start; the next would follow 3-4 s later.
             radvd -C "$conf" -p "$pid" -n -m stderr &
-          sleep 7
+            sleep 1
             kill -KILL $! 2>/dev/null || true
             wait $! || true
+            sleep 2
+          done
         done
       '';
     };