feat: add multi-engine regex flavour support

engines/cpp/README.md

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+# C++ regex engine
+
+This bridge executes patterns with the `std::wregex` implementation shipped in
+Emscripten 6.0.4's libc++. The default grammar is the C++ standard library's
+modified ECMAScript grammar; it is not presented as modern browser ECMAScript.
+
+The fixed Embind entry points accept structured values only. User patterns,
+subjects and replacement templates are passed as data and are never evaluated
+as JavaScript or C++ source. The worker remains subject to Regex Tools' hard
+timeout and result limits.
+
+Replacement is streamed through the output byte limit with libc++
+`match_results::format` default grammar. A load-time differential self-test
+compares bounded fixture output with the native API; capture amplification
+never materializes a whole per-match expansion.
+
+Build with the pinned Emscripten checkout:
+
+```sh
+node scripts/cpp-engine-pack.mjs --empp /absolute/path/to/em++
+node scripts/install-cpp-engine.mjs
+```
+
+The builder enables `wasm-ld --trace` and accepts only the reviewed archive
+set. The generated `SOURCE-MANIFEST.json` records the exact Emscripten, emsdk
+and LLVM identities; maps the selected Emscripten runtime, musl 1.2.6,
+emmalloc, compiler-rt 22.1.8, libc++ 21.1.8 and libc++abi 21.1.8 sources to
+their pinned legal files; and records that JavaScript exceptions select no
+libunwind object. The libunwind legal text is retained so that exclusion stays
+explicit and reviewable.
+
+Native positions are Unicode code-point offsets because the bridge deliberately
+uses 32-bit `wchar_t`; the TypeScript adapter preserves them and separately
+normalizes editor ranges to UTF-16.