fix(chrome): harden cssEscape for control chars + use [class~=]

Previous cssEscape only handled " and \, leaving NUL/newlines/control
chars to break out of the CSS string literal. Port the CSSOM string
serialization rules: NUL becomes U+FFFD, control chars become \HEX,
quotes/backslashes get escaped.

Class selector switched from `.x` (which would need separate identifier
escaping) to `[class~="x"]`, which is also semantically correct for
multi-class elements.

internal/driver/chrome/translate.go

@@ -2,7 +2,9 @@ package chrome
 
 import (
 	"errors"
+	"fmt"
 	"strings"
+	"unicode/utf8"
 )
 
 // TranslateStringSelector converts a legacy string selector ("id:foo",
@@ -25,7 +27,7 @@ func TranslateStringSelector(selector string) (string, bool, error) {
 	case "id", "resource-id":
 		return `[id="` + cssEscape(value) + `"]`, false, nil
 	case "class":
-		return "." + cssEscape(value), false, nil
+		return `[class~="` + cssEscape(value) + `"]`, false, nil
 	case "tag":
 		return cssEscape(value), false, nil
 	case "text":
@@ -43,20 +45,29 @@ func TranslateStringSelector(selector string) (string, bool, error) {
 	}
 }
 
-// cssEscape escapes the subset of characters that break a CSS string literal
-// inside `[attr="..."]`. Quotes and backslashes are escaped per CSSOM's
-// CSS.escape rules; other printable bytes pass through.
+// cssEscape escapes a value for use inside a CSS double-quoted string
+// (`[attr="VALUE"]`). Per the CSSOM spec for serializing strings:
+//   - U+0000 becomes U+FFFD (REPLACEMENT CHARACTER)
+//   - control characters (U+0001-U+001F, U+007F) become \HEX escapes
+//   - " and \ are escaped with a leading backslash
+//   - everything else passes through, including non-ASCII
+//
+// Callers should not pass this output into identifier contexts (class names,
+// tag names) — use an attribute selector form (`[class~="..."]`) instead.
 func cssEscape(value string) string {
 	var builder strings.Builder
 	builder.Grow(len(value))
-	for index := 0; index < len(value); index++ {
-		c := value[index]
-		switch c {
-		case '\\', '"':
+	for _, r := range value {
+		switch {
+		case r == 0:
+			builder.WriteRune(utf8.RuneError)
+		case (r >= 0x01 && r <= 0x1F) || r == 0x7F:
+			fmt.Fprintf(&builder, "\\%X ", r)
+		case r == '\\' || r == '"':
 			builder.WriteByte('\\')
-			builder.WriteByte(c)
+			builder.WriteRune(r)
 		default:
-			builder.WriteByte(c)
+			builder.WriteRune(r)
 		}
 	}
 	return builder.String()

internal/driver/chrome/translate_test.go

@@ -10,7 +10,7 @@ func TestTranslateStringSelector_KnownKeys(t *testing.T) {
 	}{
 		{"id:email", `[id="email"]`, false},
 		{"resource-id:account-name", `[id="account-name"]`, false},
-		{"class:btn-primary", `.btn-primary`, false},
+		{"class:btn-primary", `[class~="btn-primary"]`, false},
 		{"tag:button", `button`, false},
 		{"text:Sign in", `//*[normalize-space(text())="Sign in"]`, true},
 		{"desc:logout", `[aria-label="logout"]`, false},
@@ -63,3 +63,28 @@ func TestTranslateStringSelector_RejectsMissingPrefix(t *testing.T) {
 		t.Error("expected error for empty selector")
 	}
 }
+
+func TestCSSEscape_ControlCharactersAndNUL(t *testing.T) {
+	cases := []struct {
+		name  string
+		input string
+		want  string
+	}{
+		{"plain ascii", "hello", "hello"},
+		{"double quote", `say "hi"`, `say \"hi\"`},
+		{"backslash", `a\b`, `a\\b`},
+		{"newline", "a\nb", `a\A b`},
+		{"carriage return", "a\rb", `a\D b`},
+		{"form feed", "a\fb", `a\C b`},
+		{"NUL replaced", "a\x00b", "a�b"},
+		{"DEL", "a\x7Fb", `a\7F b`},
+		{"non-ascii passes through", "café", "café"},
+	}
+	for _, testCase := range cases {
+		got := cssEscape(testCase.input)
+		if got != testCase.want {
+			t.Errorf("%s: cssEscape(%q) = %q, want %q",
+				testCase.name, testCase.input, got, testCase.want)
+		}
+	}
+}