EscapeIdentifier runs for every identifier token emitted, yet it built
a StringBuilder plus a fresh result string even for the overwhelmingly
common case of an identifier with no escapable characters. A pre-scan
now returns the original instance untouched in that case, and the
surrogate-pair copy appends the two chars directly instead of cutting
a two-char substring. New unit tests pin the escaping behavior and the
identity fast path.
Assisted-by: Claude:claude-fable-5:Claude Code
Christoph Wille
519ed6e98c