This is the streaming counterpart to TextDecoder, used by sites that
process responses with `pipeThrough(new TextDecoderStream())` or that
otherwise consume a Response body as decoded text.
The transform algorithm appends each chunk's bytes to the I/O queue
and decodes them via the underlying LibTextCodec decoder, holding back
any trailing partial UTF-8 sequence so chunk boundaries don't produce
spurious replacement characters. The flush algorithm emits a single
replacement character if a partial sequence was still pending. For
non-UTF-8 encodings the underlying decoders are stateless across
calls, so each chunk is decoded in full with nothing carried over.
