docs(httpauth-firebase): fix rbac tier reference from 1 to 0

rbac is a Tier 0 module (no micro-lib dependencies). The dependency line
incorrectly cited it as Tier 1. The module's own tier (4) is unchanged —
it remains the auth layer above the transport infrastructure.

compliance_test.go

@@ -0,0 +1,33 @@
+package httpauth_test
+
+import (
+	"context"
+
+	"firebase.google.com/go/v4/auth"
+
+	httpauth "code.nochebuena.dev/go/httpauth-firebase"
+	"code.nochebuena.dev/go/rbac"
+)
+
+type mockVerifier struct{}
+
+func (m *mockVerifier) VerifyIDTokenAndCheckRevoked(_ context.Context, _ string) (*auth.Token, error) {
+	return nil, nil
+}
+
+type mockEnricher struct{}
+
+func (m *mockEnricher) Enrich(_ context.Context, _ string, _ map[string]any) (rbac.Identity, error) {
+	return rbac.Identity{}, nil
+}
+
+type mockProvider struct{}
+
+func (m *mockProvider) ResolveMask(_ context.Context, _, _ string) (rbac.PermissionMask, error) {
+	return 0, nil
+}
+
+// Compile-time interface satisfaction checks.
+var _ httpauth.TokenVerifier     = (*mockVerifier)(nil)
+var _ httpauth.IdentityEnricher  = (*mockEnricher)(nil)
+var _ httpauth.PermissionProvider = (*mockProvider)(nil)