feat: add multi-engine regex flavour support

public/engines/dotnet/engine-metadata.json

@@ -0,0 +1,166 @@
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+  "schemaVersion": 1,
+  "engine": "dotnet",
+  "engineName": "System.Text.RegularExpressions",
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+  "semanticIdentity": ".NET 10.0.10 System.Text.RegularExpressions; invariant culture; UTF-16 offsets",
+  "status": "production-native-runtime",
+  "bridge": {
+    "abiVersion": 1,
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+    "requestEncoding": "bounded JSON passed to fixed JSExport methods",
+    "userSourceEvaluation": false,
+    "replacement": "Fixed streaming parser for .NET replacement tokens ($$, $n, ${name}, $&, $`, $', $+, $_), verified against Match.Result at load",
+    "replacementOutputTransport": "bounded UTF-8 bytes encoded as canonical base64 in the JSON response",
+    "supportedApplicationFlags": [
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+      "i",
+      "m",
+      "s",
+      "n",
+      "x",
+      "r",
+      "c",
+      "b"
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+    "runtimeVersion": "10.0.10",
+    "runtimeCommit": "f7d90799ce"
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+  "toolchain": {
+    "sdkVersion": "10.0.302",
+    "sdkCommit": "35b593bebf",
+    "workload": "wasm-tools",
+    "target": "browser-wasm",
+    "command": "dotnet publish engines/dotnet/RegexTools.DotNet.csproj -c Release --no-restore -p:PathMap=<source>=/regex-tools-source -p:EmccExtraCFlags=-ffile-prefix-map=<sdk>=/dotnet-sdk-10.0.302"
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+}