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, 19 insertions, 35 deletions

diff --git a/src/programs.asm b/src/programs.asm
index a8d8b63..b5e4d8e 100644
--- a/src/programs.asm
+++ b/src/programs.asm
@@ -1,52 +1,36 @@
 ; =============================================================================
 ; programs.asm - userland program images (text-in-ROM model).
 ;
-; Each program is linked to execute from the ROMX window ($4000-$7FFF) and
-; lives in its own ROM bank. exec() maps the bank and jumps to the entry.
-;
-; To prove ROM banking really drives execution, both demo programs are ORG'd at
-; the SAME address ($4000) in DIFFERENT banks - the only thing selecting which
-; code runs is PROC_ROMB. They also read their message string out of the mapped
-; bank, so the correct bank must be live when sys_write runs.
-;
-; Programs only ever talk to the kernel via `rst $30` (the trap + all kernel
-; code live in ROM0, which is always mapped), so they never call kernel labels.
+; Programs are linked to run from the ROMX window ($4000-$7FFF) and live in
+; their own ROM banks. exec() maps the bank and jumps to the entry. Programs
+; only talk to the kernel via `rst $30` (the trap + kernel live in ROM0, always
+; mapped), so they never reference kernel labels.
 ; =============================================================================
 INCLUDE "include/gbos.inc"
 
-SECTION "prog_ping", ROMX[$4000], BANK[2]
-ProgPing:
-.loop
-    ld de, .msg
-    ld b, 1
-    ld c, SYS_WRITE
-    rst $30
-    ld c, SYS_YIELD
-    rst $30
-    jr .loop
-.msg
-    db "1"
-
-SECTION "prog_pong", ROMX[$4000], BANK[3]
-ProgPong:
-.loop
+; -----------------------------------------------------------------------------
+; PROG_WORKER: announce with 'w', then exit() with a status equal to our pid.
+; Exercises exec (runs from a ROM bank) + the exit/wait lifecycle.
+; -----------------------------------------------------------------------------
+SECTION "prog_worker", ROMX[$4000], BANK[2]
+ProgWorker:
     ld de, .msg
     ld b, 1
     ld c, SYS_WRITE
     rst $30
-    ld c, SYS_YIELD
+    ld c, SYS_GETPID
     rst $30
-    jr .loop
+    ld b, a                 ; exit status = pid (so reaped codes are distinct)
+    ld c, SYS_EXIT
+    rst $30                 ; never returns
 .msg
-    db "2"
+    db "w"
 
 ; -----------------------------------------------------------------------------
-; Program table (in ROM0, always mapped). Indexed by exec()'s program id.
-; Entry layout: bank (2 bytes, MBC5 16-bit) then entry address (2 bytes).
+; Program table (ROM0). Indexed by exec()'s program id.
+; Entry: bank (2 bytes, MBC5 16-bit) then entry address (2 bytes).
 ; -----------------------------------------------------------------------------
 SECTION "progtab", ROM0
 ProgramTable::
-    db LOW(BANK(ProgPing)), HIGH(BANK(ProgPing))
-    dw ProgPing
-    db LOW(BANK(ProgPong)), HIGH(BANK(ProgPong))
-    dw ProgPong
+    db LOW(BANK(ProgWorker)), HIGH(BANK(ProgWorker))
+    dw ProgWorker