fix(core): harden nil-safe rendering paths

- guard nil receivers and nodes in core render flows
- make Render() safe for nil input
- add compile-time Node contract for Responsive

Co-Authored-By: Virgil <virgil@lethean.io>

layout.go

@@ -81,6 +81,10 @@ func (l *Layout) blockID(slot byte) string {
 // Usage example: html := NewLayout("C").C(Text("body")).Render(NewContext())
 // Only slots present in the variant string are rendered.
 func (l *Layout) Render(ctx *Context) string {
+	if l == nil {
+		return ""
+	}
+
 	b := newTextBuilder()
 
 	for i := range len(l.variant) {
@@ -106,8 +110,12 @@ func (l *Layout) Render(ctx *Context) string {
 		b.WriteString(`">`)
 
 		for _, child := range children {
+			if child == nil {
+				continue
+			}
+
 			// Clone nested layouts before setting path (thread-safe).
-			if inner, ok := child.(*Layout); ok {
+			if inner, ok := child.(*Layout); ok && inner != nil {
 				clone := *inner
 				clone.path = bid + "-"
 				b.WriteString(clone.Render(ctx))

node.go

@@ -85,6 +85,10 @@ func El(tag string, children ...Node) Node {
 // Usage example: Attr(El("a", Text("docs")), "href", "/docs")
 // It recursively traverses through wrappers like If, Unless, and Entitled.
 func Attr(n Node, key, value string) Node {
+	if n == nil {
+		return n
+	}
+
 	switch t := n.(type) {
 	case *elNode:
 		t.attrs[key] = value
@@ -171,6 +175,9 @@ func If(cond func(*Context) bool, node Node) Node {
 }
 
 func (n *ifNode) Render(ctx *Context) string {
+	if n == nil || n.cond == nil || n.node == nil {
+		return ""
+	}
 	if n.cond(ctx) {
 		return n.node.Render(ctx)
 	}
@@ -191,6 +198,9 @@ func Unless(cond func(*Context) bool, node Node) Node {
 }
 
 func (n *unlessNode) Render(ctx *Context) string {
+	if n == nil || n.cond == nil || n.node == nil {
+		return ""
+	}
 	if !n.cond(ctx) {
 		return n.node.Render(ctx)
 	}
@@ -212,6 +222,9 @@ func Entitled(feature string, node Node) Node {
 }
 
 func (n *entitledNode) Render(ctx *Context) string {
+	if n == nil || n.node == nil {
+		return ""
+	}
 	if ctx == nil || ctx.Entitlements == nil || !ctx.Entitlements(n.feature) {
 		return ""
 	}
@@ -232,8 +245,17 @@ func Switch(selector func(*Context) string, cases map[string]Node) Node {
 }
 
 func (n *switchNode) Render(ctx *Context) string {
+	if n == nil || n.selector == nil {
+		return ""
+	}
 	key := n.selector(ctx)
+	if n.cases == nil {
+		return ""
+	}
 	if node, ok := n.cases[key]; ok {
+		if node == nil {
+			return ""
+		}
 		return node.Render(ctx)
 	}
 	return ""
@@ -259,9 +281,17 @@ func EachSeq[T any](items iter.Seq[T], fn func(T) Node) Node {
 }
 
 func (n *eachNode[T]) Render(ctx *Context) string {
+	if n == nil || n.fn == nil || n.items == nil {
+		return ""
+	}
+
 	b := newTextBuilder()
 	for item := range n.items {
-		b.WriteString(n.fn(item).Render(ctx))
+		child := n.fn(item)
+		if child == nil {
+			continue
+		}
+		b.WriteString(child.Render(ctx))
 	}
 	return b.String()
 }

render.go

@@ -3,6 +3,9 @@ package html
 // Render is a convenience function that renders a node tree to HTML.
 // Usage example: html := Render(El("main", Text("welcome")), NewContext())
 func Render(node Node, ctx *Context) string {
+	if node == nil {
+		return ""
+	}
 	if ctx == nil {
 		ctx = NewContext()
 	}

responsive.go

@@ -1,5 +1,8 @@
 package html
 
+// Compile-time interface check.
+var _ Node = (*Responsive)(nil)
+
 // Responsive wraps multiple Layout variants for breakpoint-aware rendering.
 // Usage example: r := NewResponsive().Variant("mobile", NewLayout("C"))
 // Each variant is rendered inside a container with data-variant for CSS targeting.
@@ -29,8 +32,16 @@ func (r *Responsive) Variant(name string, layout *Layout) *Responsive {
 // Render produces HTML with each variant in a data-variant container.
 // Usage example: html := NewResponsive().Variant("mobile", NewLayout("C")).Render(NewContext())
 func (r *Responsive) Render(ctx *Context) string {
+	if r == nil {
+		return ""
+	}
+
 	b := newTextBuilder()
 	for _, v := range r.variants {
+		if v.layout == nil {
+			continue
+		}
+
 		b.WriteString(`<div data-variant="`)
 		b.WriteString(escapeAttr(v.name))
 		b.WriteString(`">`)