Fix/pipe posix compliance

[valydumitru01]

Description

Rewrite pipe.rs with a POSIX-compliant, byte-bounded buffer backed by
Mutex<VecDeque<u8>>, replacing the previous mpsc channel.

Pipe fixes:

• Enforce PIPE_CAPACITY (64 KB) at byte level, not message count
• Enforce PIPE_BUF (4096) atomicity guarantee for writes
• Correct WouldBlock vs BrokenPipe error mapping
• Fix AsyncWrite which was silently using blocking tx.send()
• Three-layer wake system: Condvar (blocking threads), Waker (async
  tasks), InterestHandler (external event loop)

fd_write.rs refactor (no behavioral changes outside of pipe paths):

• Extract write_to_file, write_to_socket, write_to_pipe,
  write_to_event_notifications, write_to_buffer
• Unify PipeTx and DuplexPipe arms via write_to_pipe(&mut dyn Write)
• Introduce WriteOutcome struct and PipeWriteResult enum for type-safe
  broken-pipe signaling

Tests:

• 27 unit tests covering the full pipe contract
• E2E C tests: fork, vfork, SIGPIPE, EAGAIN, atomicity, dup2,
  drain-resume cycle, capacity bounds

Motivation

The pipe implementation was not POSIX-conformant and had multiple reported
bugs: #6279, #6148.

The fd_write refactor was done alongside for readability and to cleanly
support the new pipe error semantics (WouldBlock now surfaces as EAGAIN
through the existing map_io_err path).

[Copilot]

Pull request overview

This PR updates the WASIX pipe implementation to better match POSIX semantics (bounded capacity, PIPE_BUF atomicity, corrected error mapping, and async wakeups), and refactors fd_write to route writes through per-kind helpers and handle pipe broken-pipe signaling more explicitly. It also extends the WASIX pipe E2E C test suite to cover nonblocking behavior, capacity bounds, dup2, and atomicity.

Changes:

• Replace the previous pipe channel-based implementation with a byte-bounded VecDeque<u8> buffer and wake mechanisms for threads/async/interest handlers.
• Refactor fd_write.rs into helper functions and introduce WriteOutcome / PipeWriteResult for clearer broken-pipe handling.
• Add/extend WASIX pipe C tests for EAGAIN/EPIPE/SIGPIPE behavior, buffer bounds, and PIPE_BUF atomicity.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 16 comments.

File	Description
lib/virtual-fs/src/pipe.rs	New bounded pipe buffer implementation plus extensive Rust unit tests.
lib/wasix/src/syscalls/wasi/fd_write.rs	Dispatcher refactor and updated pipe write/error signaling path.
tests/wasix/pipes/main.c	Expanded E2E coverage for nonblocking pipes, bounds, dup2, and atomic writes.