Replace the Rust URL form-encoding callback bridge into C++ TextCodec
with a direct encoding_rs encoder.
This keeps percent-encode-after-encoding entirely in Rust and removes
liburl_rust's dependency on LibTextCodec.
It also happens to fix ISO-2022-JP URL encoding of literal U+FFFD.
The LibTextCodec reverse lookup treats generated 0xFFFD table holes as
real JIS0208 mappings, so literal U+FFFD skipped the encoder-error
path. encoding_rs treats U+FFFD as unmappable, so URL encoding emits
the required numeric character reference.
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