Ports workerd::api::node::i18n::transcode to a new Rust crate, //src/rust/i18n, selected at runtime by a new NODEJS_I18N_RUST autogate. The C++ implementation is left in place, byte for byte, as the gate-off/rollback path.

The dispatch table, output-size computation, ICU substitute-character setup, empty-input handling, ISOLATE_LIMIT checks, and truncation all move to Rust (src/rust/i18n/dispatch.rs). The underlying codecs stay the same: a new C++ shim (src/rust/i18n/shim.{h,c++}) exposes the ICU ucnv_* primitives and the four simdutf functions the C++ path already uses, so both paths call identical codecs and cannot silently diverge.

i18n::transcode gets a gate branch at the top, following the createNodeException pattern: it maps api::node::Encoding to the bridge's Encoding enum via a fromImpl overload (kj-rs/convert.h idiom) whose switch has no default arm, rejecting BASE64/BASE64URL/HEX on either side (a deliberate divergence from the unmodified C++ dispatch, which only checks the from encoding). The Rust entry point returns a jsg::v8::ffi::MaybeLocal naming a Uint8Array; ffi-inl.h gains the missing Rust-to-C++ MaybeLocal conversion (maybe_local_from_ffi), mirroring the existing local_from_ffi.

10 Rust unit tests in dispatch.rs cover every (from, to) pair, empty input, unmappable-character substitution, the ASCII->UTF16LE latin1-widening quirk, the UTF8->UTF16LE zero-estimate quirk, invalid UTF-8, odd-length UTF-16LE input, and unpaired surrogates.