feat(macos): validate dev + apple-dev presets on macOS, fix 3 cross-platform bugs

macOS port groundwork — core, server, tools, and tests now build and run on
macOS 26.5 / Apple Silicon. ctest --preset dev green 21/21 (2 consecutive runs).
apple-dev produces valid arm64 libvoicecat.a + XCFramework for the Swift Package.

Three real cross-platform bugs found and fixed (all latent on Windows/Linux):

1. test_m2_voice.cpp POSIX branch missing <netdb.h> — Linux glibc transitively
   includes it, macOS doesn't. Would fail on any strict POSIX system.

2. SIGPIPE killing processes on macOS — writing to a closed TCP socket raises
   SIGPIPE by default (doesn't exist on Windows, benign on Linux). Fixed by
   ignoring SIGPIPE in both core client init and server startup (POSIX-only,
   #ifndef _WIN32). Production fix, not just tests.

3. Use-after-free of Asio's kqueue reactor on server shutdown — the
   deterministic test_tofu_flow segfault. TcpServerConn's tls_read_loop runs on
   a blocking-I/O thread; when Server::run() returned, io_context was destroyed
   while those threads were still running. On macOS kqueue the reactor pointer
   is null'd immediately -> segfault in socket.close(). Latent on Windows IOCP
   and Linux epoll. Fix: TcpAcceptor now tracks connections; new shutdown()
   closes all + joins threads before io is destroyed; Server::stop() now closes
   acceptor + media_relay too (was just io.stop()).

Verified: dev + apple-dev presets build green, 21/21 tests pass, server starts
+ two vccli text chat over TLS (M1 on Mac), vccli --voice starts MIC stream via
CoreAudio (M2 protocol-level), vccli --list-devices enumerates CoreAudio
devices, xcodebuild -create-xcframework produces valid VoiceCatCore.xcframework.

No ABI or proto changes. Docs updated: building.md, clients/apple/README.md,
PROGRESS.md, CLAUDE.md status line.

server/src/server.cpp

@@ -1,6 +1,7 @@
 #include "server.h"
 
 #include <cstdio>
+#include <csignal>
 
 #ifdef VOICECAT_HAS_NET
 
@@ -135,13 +136,26 @@ int Server::run() {
     // Arm programmatic stop (for tests and embedders).
     {
         std::lock_guard lk(stop_mutex_);
-        stop_fn_ = [&io] { io.stop(); };
+        stop_fn_ = [&] {
+            acceptor.stop();
+            media_relay->stop();
+            io.stop();
+        };
     }
 
     // Notify caller of the actual bound port (matters when bind_port==0).
     uint16_t bound = acceptor.local_port();
     if (cfg_.on_ready) cfg_.on_ready(bound);
 
+#ifndef _WIN32
+    // POSIX/macOS: ignore SIGPIPE — writing to a disconnected client's TCP socket returns
+    // EPIPE instead of killing the server process. On macOS SIGPIPE is delivered by
+    // default (unlike Windows where the signal doesn't exist). Must be set before any
+    // async writes are posted. Process-global; safe alongside the asio::signal_set below
+    // (which only catches SIGINT/SIGTERM).
+    std::signal(SIGPIPE, SIG_IGN);
+#endif
+
     // Graceful shutdown on SIGINT/SIGTERM
     asio::signal_set signals(io, SIGINT, SIGTERM);
     signals.async_wait([&](std::error_code, int sig) {
@@ -184,6 +198,13 @@ int Server::run() {
 
     io.run();
 
+    // Close all connections and block until their TLS I/O threads have finished.
+    // MUST happen before io (and its reactor) is destroyed — the TLS read threads do
+    // blocking I/O (not async on io_context) and touch the reactor on socket close.
+    // On macOS kqueue the reactor pointer is null'd immediately on io_context destruction,
+    // causing a segfault; latent on Windows IOCP / Linux epoll where timing is more forgiving.
+    acceptor.shutdown();
+
     {
         std::lock_guard lk(stop_mutex_);
         stop_fn_ = nullptr;