refactor(core): move ESM loader hooks to CLI-only path

The jest-runner instruments code through Jest's own transformer pipeline
and monkey-patches built-ins at runtime — it never uses the ESM loader
hooks.  Extracted registerEsmLoaderHooks from registerInstrumentor,
called only from startFuzzing (the CLI entry point).  hookRequire stays
in registerInstrumentor because custom hooks loaded via import() in
initFuzzing still need coverage instrumentation through require.extensions.

Author: oetr

packages/core/core.ts

@@ -29,6 +29,7 @@ import {
 	FileSyncIdStrategy,
 	Instrumentor,
 	MemorySyncIdStrategy,
+	registerEsmLoaderHooks,
 	registerInstrumentor,
 } from "@jazzer.js/instrumentor";
 
@@ -189,7 +190,9 @@ export async function startFuzzing(
 	options: OptionsManager,
 ): Promise<FuzzingResult> {
 	registerGlobals(options);
-	await initFuzzing(options);
+	const instrumentor = await initFuzzing(options);
+	registerEsmLoaderHooks(instrumentor);
+	instrumentor.sendHooksToLoader();
 	const fuzzFn = await loadFuzzFunction(options);
 	const findingAwareFuzzFn = asFindingAwareFuzzFn(fuzzFn);
 	return startFuzzingNoInit(findingAwareFuzzFn, options).finally(() => {

packages/instrumentor/instrument.ts

@@ -237,12 +237,17 @@ export function registerInstrumentor(instrumentor: Instrumentor) {
 		(code: string, opts: TransformerOptions): string => {
 			return instrumentor.instrument(code, opts.filename)?.code || code;
 		},
-		// required to allow jest to run typescript files
-		// jest's typescript integration will transform the typescript into javascript before giving it to the
-		// instrumentor but the filename will still have a .ts extension
 		{ extensions: [".js", ".mjs", ".cjs", ".ts", ".mts", ".cts"] },
 	);
+}
 
+/**
+ * Register an ESM loader hook so that import() and static imports are
+ * instrumented too.  Only needed when modules are loaded via Node
+ * directly (CLI path).  The jest-runner uses Jest's own transformer
+ * pipeline instead and does not need the ESM loader thread.
+ */
+export function registerEsmLoaderHooks(instrumentor: Instrumentor) {
 	registerEsmHooks(instrumentor);
 }