fix(compat): version-aware systemd-networkd config key generation

Runtime detection of systemd version via networkctl --version.
No new environment variables — detection happens on the running
system at startup.

- systemd >= 256: uses IPv4Forwarding=/IPv6Forwarding= (new keys)
- systemd < 256: uses IPForward=yes (255 and earlier)
- OtherInformation= is correct for all versions (OtherConfig was
  never a valid systemd key)

Shipped .network templates use IPForward=yes for maximum
compatibility (works on 255, deprecated-but-functional on 256+).
Dynamic configs from rxnm-config-builder.sh use the runtime-
detected constants.

Author: aenertia

lib/rxnm-config-builder.sh

@@ -121,7 +121,8 @@ build_network_config() {
 
     printf "MulticastDNS=%s\nLLMNR=%s\n" "${mdns}" "${llmnr}"
     printf "LinkLocalAddressing=yes\nIPv6AcceptRA=yes\nConfigureWithoutCarrier=yes\n"
-    printf "IPForward=yes\n"
+    # shellcheck disable=SC2059
+    printf "${RXNM_NETWORKD_KEY_IPFORWARD}\n"
 
     if [ -n "$ipv6_privacy" ]; then
         printf "IPv6PrivacyExtensions=%s\n" "${ipv6_privacy}"
@@ -268,13 +269,15 @@ build_gateway_config() {
 
     if [ "$share" = "true" ]; then
         # v255.8 Compliant Forwarding (NAT is handled externally by rxnm-system)
-        printf "IPForward=yes\nIPv6SendRA=yes\n"
+        # shellcheck disable=SC2059
+        printf "${RXNM_NETWORKD_KEY_IPFORWARD}\nIPv6SendRA=yes\n"
         if [ "$ipv6_pd" != "no" ]; then printf "DHCPPrefixDelegation=yes\n"; fi
 
         printf "Address=fd00:cafe:feed::a7ca:de/64\n"
 
         printf "DHCPServer=yes\n\n[DHCPServer]\nPoolOffset=100\nEmitDNS=yes\nEmitRouter=yes\n"
-        printf "[IPv6SendRA]\nManaged=no\nOtherConfig=no\n"
+        # shellcheck disable=SC2059
+        printf "[IPv6SendRA]\nManaged=no\n${RXNM_NETWORKD_KEY_RA_OTHER}=no\n"
     else
         printf "Address=fd00:cafe:feed::a7ca:de/64\n"
         printf "IPv6AcceptRA=no\nDHCPServer=yes\n\n[DHCPServer]\nEmitDNS=yes\nEmitRouter=no\n"

lib/rxnm-constants.sh

@@ -92,6 +92,25 @@ fi
 
 : "${IWD_DBUS_MAX_KB:=512}"
 
+# --- systemd-networkd Version-Aware Config Keys ---
+# IPForward= was deprecated in systemd 256, replaced by IPv4Forwarding=/IPv6Forwarding=.
+# Detect the running networkd version and emit the correct key for dynamic configs.
+# Shipped .network templates use IPForward= (works on all versions, deprecated warning on 256+).
+if [ -z "${RXNM_NETWORKD_KEY_IPFORWARD:-}" ]; then
+    _sd_ver=""
+    if command -v networkctl >/dev/null 2>&1; then
+        _sd_ver=$(networkctl --version 2>/dev/null | awk '/^systemd / {print $2; exit}')
+    fi
+    _sd_ver="${_sd_ver:-255}"
+    if [ "$_sd_ver" -ge 256 ] 2>/dev/null; then
+        RXNM_NETWORKD_KEY_IPFORWARD="IPv4Forwarding=yes\nIPv6Forwarding=yes"
+    else
+        RXNM_NETWORKD_KEY_IPFORWARD="IPForward=yes"
+    fi
+fi
+# OtherInformation= is correct for all systemd versions (OtherConfig was never valid)
+: "${RXNM_NETWORKD_KEY_RA_OTHER:=OtherInformation}"
+
 # --- FD Reservations ---
 RXNM_FD_GLOBAL_LOCK=8
 RXNM_FD_IFACE_LOCK=9

usr/lib/systemd/network/69-wifi-p2p-go.network

@@ -8,7 +8,8 @@ Description=Rocknix Wi-Fi P2P Group Owner
 # Standard P2P subnet (Commonly used in Android/Linux P2P)
 Address=192.168.49.1/24
 DHCPServer=yes
-# IP Forwarding (v255.8 compliant). NAT is handled externally by rxnm-system.
+# IP Forwarding. NAT is handled externally by rxnm-system.
+# Uses IPForward= for systemd 255 (ROCKNIX) compatibility.
 IPForward=yes
 # Ensure local name resolution works for the group
 MulticastDNS=no

usr/lib/systemd/network/70-bluetooth-pan.network

@@ -6,7 +6,9 @@ Name=bnep*
 Description=Rocknix Bluetooth PAN Host
 Address=192.168.44.1/24
 DHCPServer=yes
-# IP Forwarding (v255.8 compliant). NAT is handled externally by rxnm-system.
+# IP Forwarding. NAT is handled externally by rxnm-system.
+# Uses IPForward= for systemd 255 (ROCKNIX) compatibility.
+# systemd 256+ deprecates this in favour of IPv4Forwarding=/IPv6Forwarding=
 IPForward=yes
 # Disabled to avoid conflict with Avahi
 MulticastDNS=no

usr/lib/systemd/network/70-wifi-ap-share.network

@@ -13,7 +13,8 @@ LinkLocalAddressing=yes
 # ULA for local IPv6
 Address=fd00:a7c:ade:50::1/64
 IPv6SendRA=yes
-# IP Forwarding (v255.8 compliant). NAT is handled externally by rxnm-system.
+# IP Forwarding. NAT is handled externally by rxnm-system.
+# Uses IPForward= for systemd 255 (ROCKNIX) compatibility.
 IPForward=yes
 # Multicast handled by Avahi
 MulticastDNS=no
@@ -27,4 +28,4 @@ EmitRouter=yes
 
 [IPv6SendRA]
 Managed=no
-OtherConfig=no
+OtherInformation=no