fix(variant): sort object keys by UTF-8 bytes like Java

[jackylee-ch]

Variant object keys are ordered with encode_utf16(), but Java orders them by
unsigned UTF-8 bytes: GenericVariantBuilder.FieldEntry.compareTo delegates to
BinaryString.compareTo, which compares (b & 0xFF) per byte.

The two disagree when a non-BMP key meets one in U+E000..=U+FFFF — UTF-16 sorts
the surrogate 0xD8xx first, UTF-8 sorts the 4-byte sequence last. A Java-written
object with, say, an emoji key beside a fullwidth key then fails to read with
Malformed Variant object key order, and the writer emits an order Java rejects.

Rust str is already UTF-8, so comparing the bytes is Java's comparison.