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, 12 insertions, 0 deletions

diff --git a/c/libc.s b/c/libc.s
index bcc8199..2e2516b 100644
--- a/c/libc.s
+++ b/c/libc.s
@@ -187,3 +187,15 @@ _progname:
 	ld	c, #21			; SYS_PROGNAME
 	rst	#0x30
 	ret
+
+	.globl _yield, _reap
+	;; void yield(void)
+_yield:
+	ld	c, #11			; SYS_YIELD
+	rst	#0x30
+	ret
+	;; unsigned char reap(void) -> A (reaped pid, or 0)
+_reap:
+	ld	c, #22			; SYS_REAP
+	rst	#0x30
+	ret