Add IPv4 socket support across backends and socket API

[lePereT]

What type of PR is this? (check all applicable)

• 🍕 Feature
• 🐛 Bug Fix
• ✅ Test

Description

Adds IPv4 socket support to fibers.io.socket and updates the FFI, nixio, and posix fd backends to accept a common sockaddr token format for UNIX and INET sockets.

Also tightens backend error handling by normalising nixio return signatures (msg, errno vs errno, msg) and fixes nixio directory mode handling so mkdir() permissions work reliably across builds.

Changes

• Export AF_INET from fd_backend.core (and backend modules)

• Add AF_INET support to fd_backend/ffi.lua (sockaddr_in, inet_pton, htons, bind/connect support)

• Extend fd_backend/nixio.lua:

  • INET/INET6 sockaddr normalisation
  • bind/connect support for UNIX/INET/INET6
  • normalised error parsing across nixio APIs
  • mkdir() permission handling using octal strings for nixio compatibility

• Extend fd_backend/posix.lua:

  • sockaddr normalisation for UNIX/INET/INET6
  • INET/INET6 metadata exports
  • more tolerant non-blocking connect handling (EINPROGRESS / EALREADY / EAGAIN)

• Extend fibers.io.socket:

  • generic :bind() / :listen()
  • listen_inet() / connect_inet() convenience helpers
  • Socket:bind_inet() / Socket:listen_inet() / Socket:connect_inet()
  • export AF_INET

• Update tests/test_io-socket.lua:

  • retain AF_UNIX roundtrip test
  • add AF_INET loopback roundtrip test (server + client, explicit source bind)
  • small test helper cleanup (read_exact, write_all, close_ok)

Related Issues, Tickets & Documents

N/A

Screenshots/Recordings

N/A (no UI changes)

Manual test

• 👍 yes

Manual test description

Ran tests/test.lua.

Added tests?

• 👍 yes

Extended tests/test_io-socket.lua with both AF_UNIX and AF_INET tests. Confirmed successful client/server roundtrip on:

• UNIX domain socket
• IPv4 loopback (127.0.0.1) with explicit client source bind

Added to documentation?

• 🙅 no documentation needed

[optional] Are there any post-deployment tasks we need to perform?

None.