catalogue: composeName handles the apex label '@' (bare public domain)

An empty label composed nothing, so a module served at the bare domain (a node's
own site) had to keep a full name — the one route the label model could not
express. The zone-file convention '@' now composes to the public domain itself,
no leading dot, so the apex is a label like any other. Test added.

Claude-Session: https://claude.ai/code/session_01LrgweAeERJYBg88c5cKDzF

internal/catalogue/declaration.go

@@ -596,6 +596,13 @@ func composeName(values map[string]any, publicDomain string) {
 	if !ok || strings.TrimSpace(label) == "" {
 		return
 	}
+	if strings.TrimSpace(label) == "@" {
+		// The apex: a module served at the bare public domain, no subdomain — the zone-file
+		// convention `@`. Composes to the domain itself, so a node's own site is a label like any
+		// other rather than the one route that must still carry a full name.
+		values["name"] = publicDomain
+		return
+	}
 	values["name"] = strings.TrimSpace(label) + "." + publicDomain
 }