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, 0 deletions

diff --git a/c/ptest.c b/c/ptest.c
new file mode 100644
index 0000000..88b7d30
--- /dev/null
+++ b/c/ptest.c
@@ -0,0 +1,19 @@
+#include "gbos.h"
+/* ptest: exercise the kernel pipe in one process - write some bytes, read them
+   back, then close the write end and confirm the reader sees EOF. */
+void main(void) {
+    unsigned char pr = pipe();
+    unsigned char pw = pr + 1;
+    int c;
+    if (pr == NOFD) { puts("pipe: none free"); nl(); return; }
+    fputc(pw, 'H'); fputc(pw, 'i'); fputc(pw, '!');
+    c = fgetc(pr); putc(c);
+    c = fgetc(pr); putc(c);
+    c = fgetc(pr); putc(c);
+    nl();
+    close(pw);                 /* last writer gone */
+    c = fgetc(pr);
+    if (c < 0) puts("EOF"); else { puts("got "); putc(c); }
+    nl();
+    close(pr);
+}