[pa-devbot]

Based on the PR for the Qt ServerSocket + test harness (merge that one first, then rebase this onto main).

Adds ServerSocket_POSIX (an accept-loop thread plus one connection
thread at a time, both from the given thread pool via plain
dispatch(), non-blocking sockets) and enables the POSIX instantiation
of the loopback tests on non-Windows.

Design

• close()/close_connection() only flip the state; the connection
  thread notices within ~1 ms and closes the socket itself, so
  close_connection() is safe to call from within a listener callback.
• The accept loop keeps running while a connection is active and
  rejects new clients via on_connect_rejected (one connection at a
  time).
• The connection task is started and published without holding the
  main lock: assigning to the task slot destroys the previous
  connection task's AsyncTask, whose destructor waits for that task
  to finish - and the task may be blocked on the main lock. Holding the
  lock across the assignment deadlocks (found while testing).

Tests: the 7 loopback scenarios (echo, zero-length send, 1 MiB
transfer, rapid reconnects, send/close race, reject second client,
port in use) now run against the POSIX backend on non-Windows in
addition to Qt. Verified: 14/14 socket tests pass on Linux, plus a
standalone POSIX harness (39 checks) and repeated stress runs.

Note (rebuild on current main): the implementation now lives in ServerSocket_POSIX.cpp, mirroring the client header/.cpp split (#1795). Also fixes an implicit size_t -> int narrowing in the send loop that -Wshorten-64-to-32 rejected on the Mac CIs.

Update (Mac CI): the 1 MiB "Large transfer" test failed on both Mac
CIs with "The client did not send all 1048576 bytes": macOS send()
with MSG_DONTWAIT readily returns after a partial write when the
kernel's send buffer is full (Linux usually does not), and
ClientSocket_POSIX::send() returned early on a partial send. It now
loops until the whole buffer is written, matching the Qt backend.
Verified with the standalone POSIX harness.

Update (formatting): fixed ServerSocket_POSIX.cpp indentation
left over from the header/.cpp split (method bodies had kept their
in-class 8-space indent). The Mutex/ConditionVariable wrapper
migration from #1787 comes in with the rebase.