Replace the generated C++ legacy codec implementations with a
small Rust wrapper around encoding_rs.
This keeps the existing LibTextCodec API while moving label lookup,
legacy decode/encode, validation, and streaming decoder state to Rust.
The generated index data and generator are no longer needed.
It also fixes several TextDecoder EOF cases due to a more correct
implementation. encoding_rs finalizes decoders according to the
Encoding Standard, so incomplete UTF-8/Big5 tails and malformed
UTF-16 surrogate tails produce the required single replacement at
end-of-queue instead of being dropped, buffered, or double-counted
by our old hand-written decoders.
