kernel: complete the process lifecycle (exit/wait/reap + bank free list)

- exit(): zombie + status, reparent orphans to init, wake blocked parent,
  schedule away forever (resources reclaimed by the reaper)
- wait(): scan for a zombie child; reap (free cart-RAM bank + PCB slot) and
  return pid/code; block PS_BLOCKED + yield if children still live; $FF if none
- cart-RAM banks are now a free list (wBankUsed bitmap): AllocRamBank/FreeRamBank
- scheduler: FindNextReady returns carry when nothing runnable; yield idles
  instead of blindly picking slot 0
- helpers: FindPcbByPid, ReparentToInit
- demo: init forks 3 workers -> exec -> exit(pid) -> wait/reap => wwwR2R3R4!
  verified bank recycling with 6 workers over 3 banks (w2w3w4w5w6w7)
- README: document the lifecycle + syscall status

1 files changed, 9 insertions, 2 deletions

diff --git a/src/kdata.asm b/src/kdata.asm
index ba395f1..5b91030 100644
--- a/src/kdata.asm
+++ b/src/kdata.asm
@@ -21,8 +21,8 @@ wTmpSlot::    DS 1
 wTmpPid::     DS 1
 wTmpSP::      DS 2
 
-; cart-RAM bank allocator (bump; bank 0 taken by the init task)
-wNextRamBank:: DS 1
+; cart-RAM bank allocator: 0 = free, 1 = in use. bank 0 is the init task's.
+wBankUsed::    DS NUM_RAM_BANKS
 
 ; scratch used by sys_fork
 wForkUserSP::    DS 2      ; parent user SP at fork entry (= Uafter)
@@ -31,3 +31,10 @@ wForkChildPid::  DS 1
 wForkParentBank::DS 1
 wForkChildBank:: DS 1
 wCopyBuf::       DS 256    ; bank-copy bounce buffer (WRAM0, always mapped)
+
+; scratch used by sys_exit / sys_wait
+wExitMyPid::     DS 1
+wExitParentPid:: DS 1
+wWaitMyPid::     DS 1
+wWaitRetPid::    DS 1
+wWaitRetCode::   DS 1