feat(local_socket): Unix core — Listen/Dial, runtime dir resolution, secure cleanup

[four-bytes-robby]

Title: feat(local_socket): Unix core — Listen/Dial, runtime dir resolution, secure cleanup

Context

First phase of interprocess-go, a Go counterpart to the Rust interprocess crate providing
local connection-oriented byte streams over a unified API. Design is D5 (Implementable):
four-file-cloud/docs/interprocess-go-concept.md.

Scope of this issue: Linux and macOS only. No Windows code, no framing subpackage — those are
Phases 2 and 4.

Deliverables

• local_socket package: Name, NameKind, Filesystem(), Namespaced(), UserScoped(),
  Listen(), Dial(), ListenOptions, DialOptions, AccessPolicy, PeerIdentity
• Returns net.Listener / net.Conn; the Listener and Conn interfaces add
  LocalSocketName() and PeerIdentity()
• Error sentinels: ErrInvalidName, ErrUnsupportedName, ErrPermissionDenied,
  ErrAlreadyInUse, ErrStaleCleanupUnsafe, ErrNoRuntimeDir, ErrPeerIdentityUnsupported
• examples/echo
• CI: ubuntu-latest, macos-latest, -race

Acceptance criteria

Concept §Implementation Phases, Phase 1 — criteria 1.1 through 1.14. All must pass.

Load-bearing ones, called out because they are easy to get subtly wrong:

• 1.3 every runtime-dir candidate is validated for owner and mode, whatever its source.
  Test: set $TMPDIR to a 0777 dir on Linux and assert it is skipped. $TMPDIR is a Darwin-only
  precedence step (Decision 2) — on Linux it conventionally means world-writable /tmp.
• 1.6 stale reclaim removes only an owned socket. Test: place a regular file at the socket
  path, assert ErrStaleCleanupUnsafe.
• 1.8 Close() unblocks Accept() with an error satisfying errors.Is(err, net.ErrClosed).
• 1.9 dial cancellation leaks no goroutine.
• 1.10 PeerIdentity() returns correct UID/GID — SO_PEERCRED (Linux), getpeereid (macOS).

Explicitly out of scope

Windows/named pipes (Phase 2) · Rust interop (Phase 3) · framing (Phase 4) · Linux abstract
namespace (Decision 3, declined permanently) · unnamed_pipe, fifo_file, message-mode pipes
(Tier 4, not ported)

Definition of done

• All 14 criteria pass, CI green on both platforms with -race
• go vet and the project linter clean
• No dependency beyond the standard library in this phase