A streamed ninja runs in its own guarded process group, as the captured one did

stream_exec ran ninja in mcpp's process group and did not register it with
the signal guard, so signalling mcpp alone left ninja running (e2e 340, the
Linux e2e shard). It now asks the bounded launcher for a group of its own,
which the guard kills when mcpp is signalled, as capture_exec's child is.

Author: speak-agent

modules/platform/src/process.cppm

@@ -1201,8 +1201,11 @@ int stream_exec(const std::vector<std::string>& argv,
     if (timed_out) *timed_out = false;
     if (spawn_error) *spawn_error = 0;
     if (argv.empty()) return 127;
+    // Its own process group, registered with the signal guard, as
+    // capture_exec's child is: a build's ninja must not outlive the mcpp that
+    // started it when that mcpp alone is signalled (e2e 340).
     auto r = dispatch_bounded(argv, extraEnv, {}, deadline, /*capture=*/true, {},
-                              std::chrono::milliseconds{0}, /*ownGroup=*/false, &on_line);
+                              std::chrono::milliseconds{0}, /*ownGroup=*/true, &on_line);
     if (!r.supported) {
         if (r.spawn_error != 0) {
             if (spawn_error) *spawn_error = r.spawn_error;