watcher: Port Change notifications list domains apart from hostnames (closes #248)

A Port Change notification's `Hosts:` line listed a port's apex domains
and hostnames together. It now has a `Domains:` line and a `Hostnames:`
line, and leaves out one that would name nothing. A name is a domain
when it is a configured domain, the rule record notifications already
use to start `Domain:` or `Hostname:`; that rule is now one method,
isDomain, which both use. The port entries saved in the state are
unchanged. README describes the new lines.

Model: opus-5-5

internal/watcher/watcher.go

@@ -542,17 +542,50 @@ func (w *Watcher) detectHostnameChanges(
 	w.detectCNAMEAddressChanges(ctx, hostname, prev, current)
 }
 
+// isDomain reports whether name is a configured apex domain, whose own
+// records are checked and saved as a hostname's are.
+func (w *Watcher) isDomain(name string) bool {
+	return slices.Contains(w.config.Domains, name)
+}
+
 // nameLine is the line a notification about name's records starts with:
-// "Domain: " and the name for a configured apex domain, whose own
-// records are checked as a hostname's are, and "Hostname: " otherwise.
+// "Domain: " and the name for a configured apex domain, and
+// "Hostname: " otherwise.
 func (w *Watcher) nameLine(name string) string {
-	if slices.Contains(w.config.Domains, name) {
+	if w.isDomain(name) {
 		return "Domain: " + name
 	}
 
 	return "Hostname: " + name
 }
 
+// portNameLines lists the names that resolve to a port's address, the
+// configured apex domains on one line and the hostnames on the next,
+// leaving out a line that would name nothing.
+func (w *Watcher) portNameLines(names []string) string {
+	var domains, hostnames []string
+
+	for _, name := range names {
+		if w.isDomain(name) {
+			domains = append(domains, name)
+		} else {
+			hostnames = append(hostnames, name)
+		}
+	}
+
+	var lines []string
+
+	if len(domains) > 0 {
+		lines = append(lines, "Domains: "+strings.Join(domains, ", "))
+	}
+
+	if len(hostnames) > 0 {
+		lines = append(lines, "Hostnames: "+strings.Join(hostnames, ", "))
+	}
+
+	return strings.Join(lines, "\n")
+}
+
 // detectCNAMEAddressChanges notifies when the addresses at the end of
 // hostname's CNAME chain differ from those the previous check saved,
 // including a change from or to none. When the previous addresses are
@@ -952,8 +985,8 @@ func (w *Watcher) checkSinglePort(
 		}
 
 		msg := fmt.Sprintf(
-			"Hosts: %s\nAddress: %s\nPort now %s",
-			strings.Join(hostnames, ", "), key, stateStr,
+			"%s\nAddress: %s\nPort now %s",
+			w.portNameLines(hostnames), key, stateStr,
 		)
 
 		w.notify.SendNotification(