Expand LAN to 10.0.0.0/22 with static/reserved/dynamic/spare zones

Splits the LAN into four /24 zones: 10.0.0.0/24 static (no DHCP, DNS
only), 10.0.1.0/24 fixed DHCP reservations by MAC, 10.0.2.0/24 dynamic
DHCP pool, 10.0.3.0/24 spare/unused. ipadm now derives the required
subnet from whether a host has a MAC, auto-assigns free IPs, and
auto-migrates a host's IP when its MAC is added/removed. Migrated the
existing archerc80 reservation from 10.0.0.2 to 10.0.1.2.

Also fixes a latent bug found while doing this: dnsmasq's SIGHUP
(`systemctl reload`) only re-reads /etc/hosts, not the conf-dir files
ipadm writes to, so config changes were silently not applied on
reload. ipadm now restarts dnsmasq instead.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tools/ipadm/interactive.go

@@ -40,19 +40,11 @@ func interactiveEdit(hosts []Host, name string, cfg netConfig) ([]Host, error) {
 		fmt.Printf("Editing existing host %q (press enter to keep a value)\n", name)
 	} else {
 		cur = Host{Name: name}
-		fmt.Printf("New host %q (press enter to leave MAC/comment empty)\n", name)
-	}
-
-	for {
-		ip := prompt("IP address", cur.IP)
-		if err := validateIP(ip, cfg.lanCIDR, cfg.poolStart, cfg.poolEnd); err != nil {
-			fmt.Fprintln(os.Stderr, "error:", err)
-			continue
-		}
-		cur.IP = ip
-		break
+		fmt.Printf("New host %q (press enter to accept defaults)\n", name)
 	}
 
+	// MAC decides which subnet the host belongs in (see netConfig), so it's
+	// asked first and used to suggest a matching free IP below.
 	for {
 		mac := prompt("MAC address (optional, '-' to clear)", macOrDash(cur.MAC))
 		if mac == "-" {
@@ -67,6 +59,24 @@ func interactiveEdit(hosts []Host, name string, cfg netConfig) ([]Host, error) {
 		break
 	}
 
+	cidr := cfg.cidrFor(cur.MAC)
+	suggested := cur.IP
+	if suggested == "" || validateIP(suggested, cidr, cfg.gatewayIP) != nil {
+		if freeIP, err := nextFreeIP(hosts, cidr, idx, cfg.gatewayIP); err == nil {
+			suggested = freeIP
+		}
+	}
+
+	for {
+		ip := prompt(fmt.Sprintf("IP address (%s)", cidr), suggested)
+		if err := validateIP(ip, cidr, cfg.gatewayIP); err != nil {
+			fmt.Fprintln(os.Stderr, "error:", err)
+			continue
+		}
+		cur.IP = ip
+		break
+	}
+
 	cur.Comment = prompt("Comment", cur.Comment)
 
 	if other := findByIP(hosts, cur.IP, idx); other >= 0 {