feat: extend user and waitanyfast

user/waitanyfast.c

@@ -0,0 +1,18 @@
+#include "ulib.h"
+
+/*
+ * Only ever spawned by waitanytest.c. Exits immediately with no sleep at
+ * all, so waitanytest.c's very first userWaitAny() call is overwhelmingly
+ * likely to find this one already Dead (the other child, waitanyslow.c,
+ * cannot possibly have finished its own userSleep by then) -- proves
+ * threadJoinAnyChild's waitScanFoundDead path picks up an already-dead
+ * child without ever needing to block at all.
+ */
+int main(void)
+{
+    print("waitanyfast: start, pid=");
+    printDec(userGetpid());
+    print("\n");
+    print("waitanyfast: exiting now\n");
+    return 44;
+}