feat: add multi-engine regex flavour support

engines/dotnet/README.md

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+# .NET regex engine
+
+This is a fixed `[JSExport]` bridge over .NET 10.0.10
+`System.Text.RegularExpressions`, built with SDK 10.0.302 for `browser-wasm`.
+It always applies `RegexOptions.CultureInvariant` for deterministic local
+results. Native match positions are UTF-16 offsets. The project uses
+WebAssembly library mode because its `[JSExport]` methods are hosted directly
+inside a module worker and do not require an executable entry point.
+
+The bridge receives one JSON value and never evaluates user input as managed or
+JavaScript source. A nine-second native regex timeout complements the app's
+shorter killable-worker deadline. Match, capture-row, pattern, subject and
+replacement-output limits are enforced in managed code as a second boundary.
+Replacement templates are parsed once by a fixed streaming implementation of
+.NET's `$$`, numbered/named-group, whole-match, left/right-context, last-group
+and whole-input tokens. Engine startup compares that implementation with
+native `Match.Result` on a parity corpus. Successful output crosses the JS
+export as bounded UTF-8 bytes in canonical base64, avoiding unbounded
+intermediate replacement strings and JSON escaping amplification.
+
+Build from the pinned SDK plus `wasm-tools` workload:
+
+```sh
+/absolute/path/to/dotnet publish engines/dotnet/RegexTools.DotNet.csproj \
+  --configuration Release
+```