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, 7 insertions, 0 deletions

diff --git a/src/programs.asm b/src/programs.asm
index 40d9034..e659cee 100644
--- a/src/programs.asm
+++ b/src/programs.asm
@@ -106,6 +106,9 @@ ProgMkdir:
 SECTION "prog_count",  ROMX[$4000], BANK[23]
 ProgCount:
     INCBIN "c/count.bin"
+SECTION "prog_ptest",  ROMX[$4000], BANK[24]
+ProgPtest:
+    INCBIN "c/ptest.bin"
 
 ; -----------------------------------------------------------------------------
 ; PROG_SH (bank 3) - the shell, now written in C (c/sh.c): parses >/</| and
@@ -192,6 +195,8 @@ ProgramTable::
     dw ProgMkdir
     db LOW(BANK(ProgCount)), HIGH(BANK(ProgCount))
     dw ProgCount
+    db LOW(BANK(ProgPtest)), HIGH(BANK(ProgPtest))
+    dw ProgPtest
 
 ; -----------------------------------------------------------------------------
 ; NameTable (ROM0) - command name -> program id, searched by sys_lookup.
@@ -242,4 +247,6 @@ NameTable::
     db PROG_MKDIR
     db "count", 0
     db PROG_COUNT
+    db "ptest", 0
+    db PROG_PTEST
     db 0