networkd: Add docs and tests for IPv4SrcValidMark=

Document the new setting in systemd.network(5) man page
and add coverage in the networkd integration tests.

Co-developed-by: Claude Opus 4.6 <noreply@anthropic.com>

test/test-network/conf/25-sysctl.network

@@ -12,5 +12,6 @@ IPv4ProxyARPPrivateVLAN=yes
 IPv6ProxyNDP=yes
 IPv6AcceptRA=no
 IPv4AcceptLocal=yes
+IPv4SrcValidMark=yes
 IPv4ReversePathFilter=no
 MulticastIGMPVersion=v1

test/test-network/systemd-networkd-tests.py

@@ -4972,6 +4972,7 @@ class NetworkdNetworkTests(unittest.TestCase, Utilities):
         self.check_ipv4_sysctl_attr('dummy98', 'proxy_arp', '1')
         self.check_ipv4_sysctl_attr('dummy98', 'proxy_arp_pvlan', '1')
         self.check_ipv4_sysctl_attr('dummy98', 'accept_local', '1')
+        self.check_ipv4_sysctl_attr('dummy98', 'src_valid_mark', '1')
         self.check_ipv4_sysctl_attr('dummy98', 'rp_filter', '0')
         self.check_ipv4_sysctl_attr('dummy98', 'force_igmp_version', '1')