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.
Reject UTF-8 second bytes outside the Encoding Standard's per-lead-byte
bounds before consuming the rest of each sequence. This keeps surrogate
and out-of-range sequences from collapsing multiple malformed bytes into
one replacement character.
Also report an odd trailing UTF-16 byte as U+FFFD through the streaming
code point path and route UTF-16 to_utf8() through the same logic. This
keeps lazy and eager script decoding aligned for bytecode cache source
hashes.
Cover the malformed UTF-8 and UTF-16 cases in LibTextCodec, TextDecoder,
and bytecode-cache source decoding tests.
Consume truncated UTF-8 tails as one malformed sequence while preserving
existing behavior for encoded surrogate code points. This keeps lazy
SourceCode decoding and bytecode cache source hashing aligned with eager
source text decoding for invalid cached script source bytes.
Continue stripping an initial UTF-8 byte order mark in
UTF8Decoder::to_utf8() so HTML parsing keeps matching the previous
String-based implementation. Cover the shared decoder behavior and the
TextDecoder API surface.
