p2: widen document fence for space and zero-width

Strip U+200B–U+200D, U+2060, U+FEFF before fencing. Match
(?i)<\s*/?\s*document so </ document> cannot sit outside the frame.

internal/digest/digest.go

@@ -577,11 +577,15 @@ func min(a, b int) int {
 	return b
 }
 
-var documentTag = regexp.MustCompile(`(?i)</?document`)
+var (
+	documentTag = regexp.MustCompile(`(?i)<\s*/?\s*document`)
+	zwChars     = regexp.MustCompile("[\u200b\u200c\u200d\u2060\ufeff]")
+)
 
 func fenceDocument(s string) string {
+	s = zwChars.ReplaceAllString(s, "")
 	return documentTag.ReplaceAllStringFunc(s, func(m string) string {
-		if len(m) >= 2 && m[1] == '/' {
+		if strings.Contains(m, "/") {
 			return "&lt;/document"
 		}
 		return "&lt;document"

internal/digest/digest_test.go

@@ -109,6 +109,17 @@ func TestDocumentTagStaysInFrame(t *testing.T) {
 	}
 }
 
+func TestFenceSpaceAndZW(t *testing.T) {
+	zw := "hello </documen\u200bt>\n"
+	out := fenceDocument(zw + "</ document>")
+	if strings.Contains(out, "</document>") || strings.Contains(strings.ToLower(out), "</ document>") {
+		t.Fatalf("unfenced: %q", out)
+	}
+	if !strings.Contains(out, "&lt;/document") {
+		t.Fatalf("missing escape: %q", out)
+	}
+}
+
 func TestFallbackOnTimeout(t *testing.T) {
 	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		time.Sleep(200 * time.Millisecond)