Fix dbg() to use lazy-initialized node modules for ESM compatibility

The previous fix moved await import() for node:fs and node:util outside
dbg() to module scope. This broke test_dbg because dbg() can be called
from --pre-js before those module-scope imports execute.

Use lazy initialization instead: declare dbg_node_fs/dbg_node_utils
early but leave them undefined. Initialize them in shell.js after fs
and utils are loaded (reusing the same imports). dbg() checks if the
modules are available and falls back to console.warn if not.

This handles all cases:
- dbg() from --pre-js (before init): uses console.warn
- dbg() after init on Node.js with pthreads: uses fs.writeSync
- dbg() in browser/non-node: uses console.warn

Author: Bartek Wrona

src/runtime_debug.js

@@ -9,29 +9,25 @@ var runtimeDebug = true; // Switch to false at runtime to disable logging at the
 
 // Used by XXXXX_DEBUG settings to output debug messages.
 #if ENVIRONMENT_MAY_BE_NODE && (PTHREADS || WASM_WORKERS)
-// Pre-load debug modules for use in dbg(). These are loaded at module scope
-// (inside async function Module()) where await is valid, since dbg() itself
-// is a regular function where await cannot be used.
-var dbg_fs, dbg_utils;
-if (ENVIRONMENT_IS_NODE) {
-  dbg_fs = {{{ makeNodeImport('node:fs', false) }}};
-  dbg_utils = {{{ makeNodeImport('node:util', false) }}};
-}
+// Node modules for dbg() output. Loaded lazily on first use since dbg() can
+// be called from --pre-js before module initialization, and await import()
+// cannot be used inside the non-async dbg() function.
+var dbg_node_fs, dbg_node_utils;
 #endif
 function dbg(...args) {
   if (!runtimeDebug && typeof runtimeDebug != 'undefined') return;
 #if ENVIRONMENT_MAY_BE_NODE && (PTHREADS || WASM_WORKERS)
   // Avoid using the console for debugging in multi-threaded node applications
   // See https://github.com/emscripten-core/emscripten/issues/14804
-  if (ENVIRONMENT_IS_NODE) {
+  if (ENVIRONMENT_IS_NODE && dbg_node_fs) {
     function stringify(a) {
       switch (typeof a) {
-        case 'object': return dbg_utils.inspect(a);
+        case 'object': return dbg_node_utils.inspect(a);
         case 'undefined': return 'undefined';
       }
       return a;
     }
-    dbg_fs.writeSync(2, args.map(stringify).join(' ') + '\n');
+    dbg_node_fs.writeSync(2, args.map(stringify).join(' ') + '\n');
   } else
 #endif
   // TODO(sbc): Make this configurable somehow.  Its not always convenient for

src/shell.js

@@ -347,6 +347,11 @@ if (ENVIRONMENT_IS_NODE) {
   var stringify = (a) => typeof a == 'object' ? utils.inspect(a) : a;
   defaultPrint = (...args) => fs.writeSync(1, args.map(stringify).join(' ') + '\n');
   defaultPrintErr = (...args) => fs.writeSync(2, args.map(stringify).join(' ') + '\n');
+#if (ASSERTIONS || RUNTIME_DEBUG || AUTODEBUG) && (PTHREADS || WASM_WORKERS)
+  // Initialize the lazy-loaded modules for dbg() now that fs/utils are available.
+  dbg_node_fs = fs;
+  dbg_node_utils = utils;
+#endif
 }
 {{{ makeModuleReceiveWithVar('out', 'print',    'defaultPrint') }}}
 {{{ makeModuleReceiveWithVar('err', 'printErr', 'defaultPrintErr') }}}