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, 41 insertions, 0 deletions

diff --git a/src/syscall.asm b/src/syscall.asm
index 04e9661..1051a4d 100644
--- a/src/syscall.asm
+++ b/src/syscall.asm
@@ -68,6 +68,7 @@ SyscallTable:
     dw sys_srecv_nb             ; 31 SRECVNB
     dw sys_pollin               ; 32 POLLIN
     dw sys_net                  ; 33 NET
+    dw sys_sleep                ; 34 SLEEP
 
 ; -----------------------------------------------------------------------------
 sys_nosys:
@@ -279,6 +280,46 @@ sys_yield:
     ret
 
 ; -----------------------------------------------------------------------------
+; sys_sleep(B = 1/64-second units) - cooperative delay driven by the DIV timer.
+; DIV (FF04) free-runs at 16384 Hz, so 256 ticks = 1/64 s. We accumulate DIV
+; deltas across SchedYields (other procs run) until enough units have elapsed.
+; -----------------------------------------------------------------------------
+sys_sleep:
+    ld a, b
+    or a
+    ret z                       ; sleep 0 = no-op
+    ld [wSleepTarget], a
+    xor a
+    ld [wSleepAcc], a
+    ld [wSleepAcc+1], a
+    ld a, [rDIV]
+    ld [wSleepLastDiv], a
+.loop
+    call SchedYield
+    ld a, [rDIV]
+    ld c, a                     ; current DIV
+    ld a, [wSleepLastDiv]
+    ld b, a                     ; previous DIV
+    ld a, c
+    ld [wSleepLastDiv], a
+    sub b                       ; A = DIV delta (mod 256)
+    ld e, a
+    ld a, [wSleepAcc]           ; acc += delta
+    add e
+    ld [wSleepAcc], a
+    ld a, [wSleepAcc+1]
+    adc 0
+    ld [wSleepAcc+1], a         ; acc high byte = elapsed 1/64-second units
+    ld b, a
+    ld a, [wSleepTarget]
+    cp b
+    jr z, .done                 ; elapsed == target
+    jr c, .done                 ; elapsed  > target
+    jr .loop
+.done
+    ret
+
+; -----------------------------------------------------------------------------
 ; sys_exit(code=B) -- become a zombie awaiting reap; never returns.
 ;   - store exit code, set PS_ZOMBIE
 ;   - reparent any children to init (so they can still be waited on)