Function canonicalize

Canonicalize a JSON-serializable value per RFC 8785.

• Object keys sorted by UTF-16 code units
• Numbers serialized per ECMAScript 7.1.12.1 (Number::toString)
• Strings escaped per RFC 8785 §3.2.2.2
• No insignificant whitespace

Throws if the value contains unsupported types (functions, undefined, symbols).