OptionalshimUrlOverride: string
Preferred shim href — the pool passes its
setDataWorkerWasmPath value. Read on the FIRST call only, which matches
the existing "set the overrides before the first worker is created"
contract of setWasmJsUrl / setDataWorkerUrl.
Compile the WASM binary once and hand the same module to every caller.
Never rejects and never hangs.