feat: add Responsive multi-variant layout compositor

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

responsive.go

@@ -0,0 +1,39 @@
+package html
+
+import "strings"
+
+// Responsive wraps multiple Layout variants for breakpoint-aware rendering.
+// Each variant is rendered inside a container with data-variant for CSS targeting.
+type Responsive struct {
+	variants []responsiveVariant
+}
+
+type responsiveVariant struct {
+	name   string
+	layout *Layout
+}
+
+// NewResponsive creates a new multi-variant responsive compositor.
+func NewResponsive() *Responsive {
+	return &Responsive{}
+}
+
+// Variant adds a named layout variant (e.g., "desktop", "tablet", "mobile").
+// Variants render in insertion order.
+func (r *Responsive) Variant(name string, layout *Layout) *Responsive {
+	r.variants = append(r.variants, responsiveVariant{name: name, layout: layout})
+	return r
+}
+
+// Render produces HTML with each variant in a data-variant container.
+func (r *Responsive) Render(ctx *Context) string {
+	var b strings.Builder
+	for _, v := range r.variants {
+		b.WriteString(`<div data-variant="`)
+		b.WriteString(v.name)
+		b.WriteString(`">`)
+		b.WriteString(v.layout.Render(ctx))
+		b.WriteString(`</div>`)
+	}
+	return b.String()
+}

responsive_test.go

@@ -0,0 +1,59 @@
+package html
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestResponsive_SingleVariant(t *testing.T) {
+	ctx := NewContext()
+	r := NewResponsive().
+		Variant("desktop", NewLayout("HLCRF").
+			H(Raw("header")).L(Raw("nav")).C(Raw("main")).R(Raw("aside")).F(Raw("footer")))
+	got := r.Render(ctx)
+
+	if !strings.Contains(got, `data-variant="desktop"`) {
+		t.Errorf("responsive should contain data-variant, got:\n%s", got)
+	}
+	if !strings.Contains(got, `data-block="H-0"`) {
+		t.Errorf("responsive should contain layout content, got:\n%s", got)
+	}
+}
+
+func TestResponsive_MultiVariant(t *testing.T) {
+	ctx := NewContext()
+	r := NewResponsive().
+		Variant("desktop", NewLayout("HLCRF").H(Raw("h")).L(Raw("l")).C(Raw("c")).R(Raw("r")).F(Raw("f"))).
+		Variant("tablet", NewLayout("HCF").H(Raw("h")).C(Raw("c")).F(Raw("f"))).
+		Variant("mobile", NewLayout("C").C(Raw("c")))
+
+	got := r.Render(ctx)
+
+	for _, v := range []string{"desktop", "tablet", "mobile"} {
+		if !strings.Contains(got, `data-variant="`+v+`"`) {
+			t.Errorf("responsive missing variant %q in:\n%s", v, got)
+		}
+	}
+}
+
+func TestResponsive_VariantOrder(t *testing.T) {
+	ctx := NewContext()
+	r := NewResponsive().
+		Variant("desktop", NewLayout("HLCRF").C(Raw("d"))).
+		Variant("mobile", NewLayout("C").C(Raw("m")))
+
+	got := r.Render(ctx)
+
+	di := strings.Index(got, `data-variant="desktop"`)
+	mi := strings.Index(got, `data-variant="mobile"`)
+	if di < 0 || mi < 0 {
+		t.Fatalf("missing variants in:\n%s", got)
+	}
+	if di >= mi {
+		t.Errorf("desktop should appear before mobile (insertion order), desktop=%d mobile=%d", di, mi)
+	}
+}
+
+func TestResponsive_ImplementsNode(t *testing.T) {
+	var _ Node = NewResponsive()
+}