fix(process): await Swift frontend lifetime pipe (#7948)

src/process/DarwinProcessRunTokenReader.ts

@@ -523,6 +523,7 @@ export const createDarwinProcessRunTokenReader = (
           maxBuffer: 1024 * 1024,
           signal,
           stopSignal: SWIFTC_INTERRUPT,
+          lifetimePipe: true,
           env: compilerEnvironment,
         });
       } catch (cause: unknown) {
@@ -535,9 +536,9 @@ export const createDarwinProcessRunTokenReader = (
     } catch (cause: unknown) {
       operation.primaryFailure = { cause };
       if (operationRoot !== undefined) {
-        // execFileOutput has settled, so inherited compiler pipes are closed.
-        // That is not proof for a child that did not inherit them; this owner
-        // does not signal processes it did not launch.
+        // The extra pipe keeps settlement pending for Swift frontend writers
+        // that inherit it after the driver and its standard streams close.
+        // It cannot establish completion of a child that did not inherit it.
         try {
           await cleanupRoot(operationRoot);
         } catch (cleanup: unknown) {