Log build-gap typecheck spawn failures

scripts/run-typecheck-build-gaps.mjs

@@ -16,6 +16,11 @@ function run(command, args) {
     stdio: "inherit",
   });
 
+  if (result.error) {
+    console.error(`[typecheck:build-gaps] Failed to spawn ${command}: ${result.error.message}`);
+    process.exit(1);
+  }
+
   if (result.status !== 0) {
     process.exit(result.status ?? 1);
   }