fix: deterministic attributes and thread-safe nested layouts

Sort attribute keys for deterministic HTML output (I-1).
Clone nested layouts before mutating path in Render (I-3),
preventing data races when the same tree is rendered concurrently.

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

layout.go

@@ -100,9 +100,12 @@ func (l *Layout) Render(ctx *Context) string {
 		b.WriteString(`">`)
 
 		for _, child := range children {
-			// Propagate path to nested layouts.
+			// Clone nested layouts before setting path (thread-safe).
 			if inner, ok := child.(*Layout); ok {
-				inner.path = bid + "-"
+				clone := *inner
+				clone.path = bid + "-"
+				b.WriteString(clone.Render(ctx))
+				continue
 			}
 			b.WriteString(child.Render(ctx))
 		}

node.go

@@ -1,6 +1,7 @@
 package html
 
 import (
+	"sort"
 	"strings"
 
 	i18n "forge.lthn.ai/core/go-i18n"
@@ -76,11 +77,17 @@ func (n *elNode) Render(ctx *Context) string {
 	b.WriteByte('<')
 	b.WriteString(n.tag)
 
-	for key, val := range n.attrs {
+	// Sort attribute keys for deterministic output.
+	keys := make([]string, 0, len(n.attrs))
+	for k := range n.attrs {
+		keys = append(keys, k)
+	}
+	sort.Strings(keys)
+	for _, key := range keys {
 		b.WriteByte(' ')
 		b.WriteString(key)
 		b.WriteString(`="`)
-		b.WriteString(escapeAttr(val))
+		b.WriteString(escapeAttr(n.attrs[key]))
 		b.WriteByte('"')
 	}