Avoid mutating global fetch in Node.js

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Summary

• preserve Node's native global fetch
• load the local WebAssembly binary without installing global shims
• preserve a preassembled wasmBinary
• add a regression test

Testing

• npm test on Node.js 20.9.0
• exact initEngine("lite-single") reproduction from initEngine() (Node) silently clobbers globalThis.fetch #119
• Emscripten 3.1.7 lite single-threaded and multithreaded builds on Node.js 18.19.1
• real engine initialization, native fetch(), and UCI searches for both builds

Closes #119

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Related: this also covers the Node 18 local-WASM loading problem reported in #64. I reproduced the original failure with Stockfish.js 15 on Node 18.19.1. Current master already works with the normal string path because of the global fetch/XMLHttpRequest workaround that this PR replaces; the PR version preserves that behavior while keeping native fetch intact and leaving globalThis.XMLHttpRequest untouched. The pathToFileURL() workaround shown in #64 is no longer needed.