Treat trailing UTF-8 prefixes with an invalid second byte as complete
input for streaming decode, so replacement characters are emitted in the
current chunk instead of being held until later input or finish. Keep
valid incomplete prefixes buffered across chunk boundaries.
Keep TextDecoderStream from holding continuation bytes after an invalid
lead byte at a chunk boundary. Add LibTextCodec and TextDecoderStream
coverage for invalid tails, valid split sequences, EOF partials,
surrogate sequences, and malformed continuation tails.
