kernel: sys_sleep - a cooperative delay off the DIV timer (+ ping pacing)

There was no time source at all (IRQ vectors just reti; scheduler is purely
cooperative). The DIV register (FF04) free-runs at 16384 Hz regardless of
interrupts, so sys_sleep accumulates DIV deltas across SchedYields (other procs
keep running) until the requested number of 1/64-second units elapse.

- SYS_SLEEP(34): B = 1/64s units; libc gsleep(units) / msleep(ms) wrappers.
- ping now msleep(800) between echoes, so it paces like real ping instead of
  blasting all four at once.

Verified real-time (capped emulator): replies land ~0.85s apart. In --uncapped
runs the delay is GB-time (fast wall-clock), as expected.

1 files changed, 1 insertions, 0 deletions

diff --git a/c/ping.c b/c/ping.c
index c98f106..68c4c86 100644
--- a/c/ping.c
+++ b/c/ping.c
@@ -47,6 +47,7 @@ void main(void) {
         } else {
             puts("  seq="); putu(seq); puts(" timeout"); nl();
         }
+        if (seq < 4) msleep(800);      /* pace like real ping */
     }
     puts("-- "); putu(got); puts("/4 received"); nl();
     net_close(s);