feat: add multi-engine regex flavour support

engines/rust/README.md

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+# Rust regex WebAssembly engine
+
+This directory builds the actual Rust `regex` crate 1.13.1 for
+`wasm32-unknown-unknown` with Rust 1.97.1 and wasm-bindgen 0.2.126.
+
+The fixed bridge accepts JSON values only. The adapter measures the exact
+encoded request size, and the native cap includes JSON's worst-case sixfold
+escaping for every accepted string field. It exposes native UTF-8 byte offsets,
+capture names, iteration, and `Captures::expand` replacement syntax. It does
+not evaluate user values as Rust or JavaScript source.
+
+Replacement uses a bounded streaming implementation of
+`Captures::expand` grammar. Load-time differential fixtures compare it with
+the native API, and capture references stop expanding as soon as the UTF-8
+output cap is reached.
+The bounded bytes cross the JSON bridge as base64, so control-heavy output has
+a fixed 4:3 transport expansion instead of JSON's content-dependent escaping.
+
+Flags `i`, `m`, `s`, `U`, `x`, and `R` map directly to `RegexBuilder`.
+Unicode is the crate's native default; the accepted `u` flag explicitly
+affirms that default. `g` selects the crate's native iterator.
+
+The crate deliberately excludes look-around and backreferences. Compile-size,
+DFA-cache, pattern, subject, match, capture-row, and output limits are enforced
+inside the module. Replacement output is clipped only at a UTF-8 boundary.