jobs: background execution (&), non-blocking reap, spin demo

- sys_reap: nohang reap of a finished zombie child (-> pid or 0).
- shell: 'cmd &' launches in the background (prints [pid], no wait); reaps
  finished jobs at the prompt ([pid done]). Foreground now waits for its own
  child specifically (loops wait(), reporting bg completions meanwhile) so kill
  reports the right pid.
- spin: a process that yields forever - a target for ps/kill.
- verified: spin &; ps shows it; kill <pid> removes just that one; immortal
  init; self-finishing bg worker reaped with [pid done].

1 files changed, 6 insertions, 0 deletions

diff --git a/c/spin.c b/c/spin.c
new file mode 100644
index 0000000..2a3d4e8
--- /dev/null
+++ b/c/spin.c
@@ -0,0 +1,6 @@
+#include "gbos.h"
+/* spin: a harmless long-running process that just yields forever.
+   Useful as a target for `ps` and `kill`. */
+void main(void) {
+    for (;;) yield();
+}