feat: extend user and waitanyfast

user/waitanyslow.c

@@ -0,0 +1,23 @@
+#include "ulib.h"
+
+/*
+ * Only ever spawned by waitanytest.c. Sleeps a real, non-trivial amount
+ * before exiting -- long enough that waitanytest.c's own userWaitAny()
+ * calls cannot possibly race past it by luck: its first call happens
+ * essentially immediately after spawning both children (see
+ * waitanytest.c's own file comment), so this thread genuinely cannot be
+ * Dead yet by the time that first call runs. That is what forces
+ * waitanytest.c's SECOND userWaitAny() call to take threadJoinAnyChild's
+ * waitScanNoneDead branch and actually block through the real
+ * retry-syscall path, rather than getting lucky and finding it already
+ * dead too.
+ */
+int main(void)
+{
+    print("waitanyslow: start, pid=");
+    printDec(userGetpid());
+    print("\n");
+    userSleep(150);
+    print("waitanyslow: exiting now\n");
+    return 33;
+}