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, 14 insertions, 0 deletions

diff --git a/src/fs.asm b/src/fs.asm
index a4ad77e..1c6d299 100644
--- a/src/fs.asm
+++ b/src/fs.asm
@@ -761,6 +761,13 @@ sys_open::
 ; =============================================================================
 sys_close::
     ld a, b
+    call is_pipe_fd
+    jr c, .notpipe
+    call pipe_close
+    xor a
+    ret
+.notpipe
+    ld a, b
     cp OF_MAX
     jr nc, .done
     ld [wFsSlot], a             ; save fd
@@ -787,6 +794,9 @@ sys_close::
 ; =============================================================================
 sys_getb::
     ld a, b
+    call is_pipe_fd
+    jp nc, pipe_read            ; A = pipe read fd
+    ld a, b
     cp OF_MAX
     jr nc, .eof
     call OFPtr
@@ -868,6 +878,10 @@ sys_getb::
 sys_putb::
     ld a, e
     ld [wFsByte], a
+    ld [wPipeByte], a
+    ld a, b
+    call is_pipe_fd
+    jp nc, pipe_write           ; A = pipe write fd, byte in wPipeByte
     ld a, b
     cp OF_MAX
     jr nc, .done