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
 }
 

internal/catalogue/routing_test.go

@@ -48,6 +48,22 @@ func TestALabelComposesWithTheNodesPublicDomain(t *testing.T) {
 	}
 }
 
+func TestTheApexLabelComposesToTheBareDomain(t *testing.T) {
+	got, err := Resolve(shelf(proxy(), labelled("board", "@", 4000)),
+		[]string{"board"}, withDomain("example.tld"), World{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	given := received(t, mustDeclare(t, got))
+	if len(given) != 1 {
+		t.Fatalf("the proxy was told about %d of 1: %v", len(given), given)
+	}
+	// `@` is the zone-file apex: served at the bare public domain, no subdomain, no leading dot.
+	if given[0].Values["name"] != "example.tld" {
+		t.Fatalf("the apex label did not compose to the bare domain: %v", given[0].Values)
+	}
+}
+
 func TestOneNodeSettingMovesTheCatalogueBetweenDomains(t *testing.T) {
 	// The ADR's name-agnostic check: the same catalogue resolves against two different public
 	// domains by changing one node setting and nothing else.