feat: add multi-engine regex flavour support

engines/ruby/README.md

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+# Ruby regular-expression engine
+
+Regex Tools executes Ruby expressions in the actual CRuby 4.0.0 engine built
+for `wasm32-wasi` by ruby.wasm. The reviewed runtime is the minimal
+`ruby.wasm` from `@ruby/4.0-wasm-wasi@2.9.3-2.9.4`; the JavaScript host API is
+`@ruby/wasm-wasi@2.9.3-2.9.4`.
+
+The fixed `bridge.rb` module is evaluated once during worker start. Pattern,
+flags, subject, limits and replacement travel through a bounded,
+length-prefixed file in the worker-local WASI memory filesystem. Bounded inert
+JSON returns metadata only; replacement output is written as UTF-8 to a
+separate bounded worker-local file. No user-controlled value is evaluated or
+interpolated as Ruby or JavaScript source, and the bridge does not use the
+ruby.wasm `JS.eval` conversion path.
+
+CRuby performs compilation and matching and reports character offsets, which
+the adapter exposes as Unicode code-point native offsets and normalizes to
+browser UTF-16 ranges. Native `String#gsub`/`String#sub` block iteration drives
+replacement matching without materializing expanded output. A fixed streaming
+implementation of CRuby 4.0.0's `rb_reg_regsub` grammar handles `\1` through
+`\9`, `\k<name>`, `\0`, `\&`, ``\` ``, `\'`, `\+`, and `\\`; a load-time
+bounded fixture verifies parity with CRuby. The expander stops on a valid UTF-8
+boundary at the configured byte limit. Unpaired browser UTF-16 surrogates are
+rejected before request encoding. A workbench timeout, cancellation,
+supersession or crash terminates the dedicated worker and its complete
+WebAssembly heap.
+
+The deterministic pack contains only:
+
+- `ruby.wasm`;
+- `engine-metadata.json`;
+- `SHA256SUMS`;
+- ruby.wasm's exact `LICENSE.txt`;
+- ruby.wasm's complete `NOTICE.txt`, including CRuby and bundled third-party
+  notices; and
+- the exact browser WASI shim and `tslib` license texts required by the
+  JavaScript host bundle.