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.

docs/building.md

@@ -17,7 +17,7 @@ and *how to drive the binaries by hand*.
 | `release` | `build/release` | vcpkg | Release | ON | ON | ON | no | all | Optimized build with the full test suite. Use to run tests against optimized code, profile, or catch optimizer-sensitive bugs. Symbols kept (not stripped) so stack traces and profiling remain useful. |
 | `server-release` | `build/server-release` | vcpkg | Release | ON | ON | OFF | **yes** | all | Production-shaped build for deployment. Optimized + stripped binaries (`-s`), no tests. This is what you'd ship/run — see [docs/deployment.md](deployment.md). |
 | `windows-client` | `build/windows-client` | vcpkg | Release | OFF | OFF | OFF | no | Windows | Produces a redistributable `voicecat.dll` for the C# WinForms client (M4). Static MinGW runtime — no `libgcc_s_seh-1.dll` etc. See [clients/windows/README.md](../clients/windows/README.md). |
-| `apple-dev` | `build/apple-dev` | vcpkg | Release | OFF | OFF | OFF | no | macOS | **Scaffolding** — static `libvoicecat.a` for the Swift Package / XCFramework (macOS slice). Not yet CI-validated; build on macOS to verify. See [clients/apple/README.md](../clients/apple/README.md). |
+| `apple-dev` | `build/apple-dev` | vcpkg | Release | OFF | OFF | OFF | no | macOS | Static `libvoicecat.a` for the Swift Package / XCFramework (macOS slice). Validated on macOS 26.5 / Apple Silicon — builds green, produces valid arm64 `.a` + XCFramework. See [clients/apple/README.md](../clients/apple/README.md). |
 | `apple-ios` | `build/apple-ios` | vcpkg | Release | OFF | OFF | OFF | no | macOS→iOS | **Scaffolding** — cross-compiled static `libvoicecat.a` for iOS device (`arm64-ios`). One XCFramework slice. Not yet CI-validated. |
 | `apple-ios-sim` | `build/apple-ios-sim` | vcpkg | Release | OFF | OFF | OFF | no | macOS→iOS sim | **Scaffolding** — cross-compiled static `libvoicecat.a` for iOS simulator (`arm64-ios-sim`). One XCFramework slice. Not yet CI-validated. |
 
@@ -207,8 +207,10 @@ symbol tables from the binaries, producing smaller executables suitable for dist
 ## 6. Apple platform builds (scaffolding)
 
 The `apple-dev`, `apple-ios`, and `apple-ios-sim` presets produce static `libvoicecat.a`
-slices for the Swift Package / XCFramework. They are **scaffolding** — not yet CI-validated
-and won't build on Windows. Build on macOS:
+slices for the Swift Package / XCFramework. The `apple-dev` preset (macOS slice) is
+**validated** — it builds green on macOS 26.5 / Apple Silicon and produces a valid arm64
+`.a` + XCFramework. The iOS cross-compile presets (`apple-ios`, `apple-ios-sim`) are still
+**scaffolding** — not yet CI-validated. Build on macOS:
 
 ```bash
 # macOS slice (arm64-osx on Apple Silicon, x64-osx on Intel)