[PHP] test: verify AF_UNIX listener queues across fork and exec

[brandonpayton]

Purpose

Add durable cross-host regression coverage for the POSIX listener behavior now
owned by Batch 2: an AF_UNIX listener inherited across fork() keeps one
accept queue, and exec() closes its FD_CLOEXEC alias without losing a
surviving alias or an already-queued connection.

The original #745 production patch is not replayed. Its socket-wire and accept
queue intent was superseded by the reviewed in-place implementation already in
the aggregate (e3c7ba3a9 and 9def16b2a).

Reviewed result

• Adds one fork-instrumented guest fixture that:
  • creates a SOCK_CLOEXEC listener and a non-CLOEXEC dup();
  • forks and queues a connection before the child calls execve();
  • closes every parent listener alias, making the child the sole owner;
  • proves the CLOEXEC alias is EBADF after exec while the surviving alias
    remains a listening socket;
  • accepts pre-exec and post-exec connections from the same FIFO queue,
    exactly once, and exchanges data over both.
• Runs the same guest through the centralized Node host and real Chromium host.
• Makes the Chromium check fail on console errors, page errors, failed
  requests, or HTTP error responses as well as incorrect guest results.
• Corrects stale SocketInfo comments to describe the existing shared listener
  queue and clone-reset behavior.
• Drops every original change to exec serialization, socket syscalls, Wasm
  exports, and host poll logging.

This changes no runtime behavior, ABI surface, package artifact, or VFS image.

Review view

Accept into Batch 2 as regression coverage via rebase merge. The production
behavior is already supplied by the aggregate; this PR makes it durable across
the real guest, Node, and browser paths. It does not independently make #871
ready for main, and the aggregate ABI-17 and full ready-to-merge gates remain.

Validation

On the exact reviewed commit:

• cargo test -p kandelo --target aarch64-apple-darwin --lib — 1076/1076
  passed.
• cd host && npx vitest run test/unix-listener-exec.test.ts — 1/1 passed.
• CI=true KANDELO_PLAYWRIGHT_PORT=5545 npx playwright test test/unix-listener-exec.spec.ts --project=chromium --workers=1 — real
  Chromium, 1/1 passed. Vite reported missing optional Node-demo artifacts
  during its dependency scan; the isolated test-runner route and guest ran
  successfully with no captured browser/runtime diagnostics.
• bash scripts/check-abi-version.sh — passed; it reported only the
  aggregate's already-known backward-compatible additive exports.
• Prettier on the three touched TypeScript files — passed.
• git diff --check — clean.

Earlier on the same repaired behavior tree, scripts/build-musl.sh and
bash build.sh also completed. The full host, libc, POSIX, Sortix, PHPT,
canonical browser, and manual ./run.sh browser gates were not run for this
test-only constituent; they remain aggregate-only under the locked Batch 2
policy.