station/hostapd: Resolve station IPs via udhcpd leases and ARP fallback

The hostapd backend never populated IPs: NewDHCPCorrelator was defined
but never instantiated, and even when it was, the parser only handled
ISC dhcpd's text format. On a BusyBox-based router using udhcpd, every
device showed up with an empty IP.

Two fixes:

- Add a udhcpd binary lease parser. The format is documented in
  busybox/networking/udhcp/dhcpd.{h,c}: an 8-byte big-endian unix-time
  header followed by 36-byte dyn_lease records (expires, IP, MAC,
  20-byte hostname, 2-byte pad). ParseLeases auto-detects the format
  by inspecting the header so the same code path handles both udhcpd
  and ISC text leases.

- Wire the DHCPCorrelator into Backend.Initialize and have it merge
  two sources: ARP first (universal IP fallback for any station that
  has been talked to) and DHCP leases on top (authoritative, carries
  the hostname). ARP fills the gap when leases are missing or the
  station uses a static IP; DHCP wins on conflict.

Default DHCPLeasesPath updated to /var/lib/udhcpd/udhcpd.leases — the
common BusyBox path. Configurable as before.

internal/station/arp/parser.go

@@ -17,6 +17,13 @@ type ARPEntry struct {
 	Device    string
 }
 
+// ParseTable is the exported entry point for the /proc/net/arp parser.
+// Other packages (e.g. the hostapd correlator) use it as a universal IP
+// source for stations whose DHCP lease isn't available.
+func ParseTable(path string) ([]ARPEntry, error) {
+	return parseARPTable(path)
+}
+
 // parseARPTable reads and parses ARP table from /proc/net/arp format
 func parseARPTable(path string) ([]ARPEntry, error) {
 	var entries []ARPEntry
@@ -26,7 +33,7 @@ func parseARPTable(path string) ([]ARPEntry, error) {
 		return entries, err
 	}
 
-	for _, line := range strings.Split(string(content), "\n") {
+	for line := range strings.SplitSeq(string(content), "\n") {
 		fields := strings.Fields(line)
 		if len(fields) > 5 {
 			var entry ARPEntry