fix(process): retain failures through finalization

src/process/ExecFileOutput.ts

@@ -220,6 +220,7 @@ const startStoppableCommand = (
   }
   let stopped: "abort" | "timeout" | "output-limit" | undefined;
   let processError: Error | null = null;
+  let rootFailed = false;
   let outputError: Error | undefined;
   let deadline: NodeJS.Timeout | undefined;
   let grace: NodeJS.Timeout | undefined;
@@ -233,7 +234,7 @@ const startStoppableCommand = (
   });
   // The first stop request names the failure; a later one is not resent.
   const stop = (reason: "abort" | "timeout" | "output-limit"): void => {
-    if (stopped !== undefined) return;
+    if (stopped !== undefined || rootFailed || processError !== null) return;
     stopped = reason;
     child.kill(stopSignal);
     grace = setTimeout(() => child.kill("SIGKILL"), STOP_GRACE_MS);
@@ -268,6 +269,11 @@ const startStoppableCommand = (
   child.on("error", (error: Error) => {
     processError ??= error;
   });
+  child.on("exit", (code, exitSignal) => {
+    // A failed root has completed its outcome even while descendants retain
+    // its pipes. Finalization cancellation cannot replace that failure.
+    rootFailed = code !== 0 || exitSignal !== null;
+  });
   child.on("close", (code, exitSignal) => {
     signal?.removeEventListener("abort", onAbort);
     clearTimeout(deadline);
@@ -290,7 +296,9 @@ const startStoppableCommand = (
         ? new ExecFileOutputCancelledError(signal.reason)
         : stopped === "timeout" && timeout !== undefined
           ? timeoutError(command, timeout, processError)
-          : (outputError ?? processError);
+          : rootFailed && stopped === undefined
+            ? processError
+            : (outputError ?? processError);
     if (failure !== null && outputError !== undefined)
       Reflect.set(failure, "outputTruncated", true);
     complete(failure, out, diagnostic);