kernel: implement FIFO (named pipe) semantics — fixes process substitution / brew config

[brandonpayton]

Root cause

mknod(S_IFIFO) / mkfifo created a regular file with no pipe semantics — the mknod dispatcher stripped the S_IFIFO type bits (mode & 0o7777) and made a plain file. A concurrent reader of that "FIFO" saw an empty regular file and got a premature EOF.

This breaks bash process substitution read < <(cmd): bash mknod()s a FIFO in $TMPDIR (/tmp/sh-np.XXXXXX) and reads it while the writer subshell is still producing output. Traced end-to-end in a Kandelo guest: the reader's read returned 0 because the temp file was an empty S_IFREG (fstat fifo=false file=true size=0).

It blocked Homebrew in Kandelo: brew config/doctor parse git --version via read < <(printf …) and discover ruby via while read < <(which -a ruby). Both got empty output → "Please update your system Git" and "No Homebrew ruby available for wasm32". Both collapse to this one bug.

Fix — real named kernel pipes

• crate::fifo: registry mapping a FIFO's canonical path → backing pipe index.
• mknod/mknodat(S_IFIFO) register a kernel pipe (PipeBuffer::new_fifo, no endpoints open) instead of creating a regular file.
• open() connects a read or write end to that shared pipe (add_reader/add_writer) → real block/EAGAIN/EOF semantics. Fork inheritance works through the existing negative-host_handle FileType::Pipe OFD path (no new fork logic).
• A FIFO reader with no writer yet blocks (EAGAIN) rather than reporting EOF, until the first writer connects — fixes the reader-opens-before-writer race.
• stat/lstat/newfstatat report S_IFIFO for registered FIFOs (a FIFO has no VFS inode).
• unlink/unlinkat remove the name and free the pipe once idle; FIFO pipes are exempt from is_fully_closed so they persist until unlinked.

Validation

• Process-substitution repro: read a < <(sleep 0.5; printf X); echo [$a] → [X] (was []).
• brew config --no-install-from-api now exits 0 inside Kandelo (previously failed on git-version + portable-ruby). API-mode config now proceeds past both blockers to a separate downstream curl issue.
• 964 kernel unit tests pass, incl. new test_fifo_named_pipe_semantics (EAGAIN-before-writer, data after write, EAGAIN-when-empty-open, EOF after writer close, stat S_IFIFO, unlink).
• No ABI change — reuses FileType::Pipe; no new syscalls or repr(C) structs; abi/snapshot.json unchanged.

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