kernel+sh: real anonymous pipes (concurrent, streaming, SIGPIPE)

Replace the shell's temp-file pipe hack with proper in-kernel FIFOs.

Kernel (src/pipe.asm, new):
- A small pool of bounded ring buffers (PIPE_MAX=2, 64 B each) with
  writers/readers refcounts. Pipe fds are $F0+idx*2 (read) / +1 (write);
  $FF stays "console".
- SYS_PIPE allocates one (writers=readers=1) and returns the read fd
  (write = read+1). getb/putb/close dispatch pipe fds here; sys_exit drops
  the refcounts held as PROC_STDIN/PROC_STDOUT.
- Blocking with SchedYield, which is the flow control: read blocks while
  empty with a writer (EOF once writers hit 0), write blocks while full
  with a reader, and if the last reader is gone the writer is killed
  (SIGPIPE -> exit 141). Cooperative-scheduler friendly.

Shell (c/sh.c):
- run_pipeline(): split on '|', make a pipe between adjacent stages, and
  fork ALL stages concurrently (no wait between), wiring stdin/stdout;
  then wait for all. Per-stage >/< still honored; orphaned pipe ends are
  closed on a lookup miss so EOF/EPIPE propagate.
- Drop the __pipe temp file and its 2 KB / serialized limits.

libc: pipe(). New c/ptest.c exercises the FIFO (write, read back, EOF).

Now works (old version couldn't): multi-stage a|b|c; streaming beyond 2 KB
(count 120 | wc = 3372 bytes through a 64 B buffer); early-exit SIGPIPE
(count 200 | true kills count instead of hanging/overflowing a file).

1 files changed, 19 insertions, 1 deletions

diff --git a/src/syscall.asm b/src/syscall.asm
index cb2c996..c602097 100644
--- a/src/syscall.asm
+++ b/src/syscall.asm
@@ -62,6 +62,7 @@ SyscallTable:
     dw sys_mkdir                ; 25 MKDIR
     dw sys_chdir                ; 26 CHDIR
     dw sys_opendir              ; 27 OPENDIR
+    dw sys_pipe                 ; 28 PIPE
 
 ; -----------------------------------------------------------------------------
 sys_nosys:
@@ -256,7 +257,24 @@ sys_yield:
 ;   - wake our parent if it is blocked in wait()
 ;   - schedule away forever (RAM bank + PCB slot are freed by the reaper)
 ; -----------------------------------------------------------------------------
-sys_exit:
+sys_exit::
+    ; release any pipe ends held as stdin/stdout (so EOF/free fire)
+    ld a, [wCurProc]
+    add PROC_STDIN
+    ld l, a
+    ld a, [wCurProc+1]
+    adc 0
+    ld h, a
+    ld a, [hl]
+    call pipe_close
+    ld a, [wCurProc]
+    add PROC_STDOUT
+    ld l, a
+    ld a, [wCurProc+1]
+    adc 0
+    ld h, a
+    ld a, [hl]
+    call pipe_close
     ; PROC_STATE = PS_ZOMBIE
     ld a, [wCurProc]
     ld l, a