chore: remove skeleton build mode and stub #ifdef scaffolding

Drop the M0 no-deps skeleton preset and all VOICECAT_HAS_NET/AUDIO/OPUS/NS
guards that it required. Every subsystem is fully implemented; the stub
#else paths were dead code that added noise to every header and source file.

- CMakePresets.json: remove skeleton configure/build/test entries
- CMakeLists.txt (root/core/tests): remove VOICECAT_USE_VCPKG_DEPS option
  and guards; all targets now build unconditionally
- 17 C++ source files: unwrap HAS_* guards, delete stub #else blocks
- apm_processor.cpp: delete ApmPassthrough no-op class; create() always
  returns RnnoiseProcessor
- 18 test files: remove HAS_* guards and stub int main() skip bodies
- docs/building.md: remove skeleton from preset table and prose

VOICECAT_HAS_LOOPBACK (Windows WASAPI loopback platform gate) unchanged.
29/29 ctest green.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/src/server.cpp

@@ -3,8 +3,6 @@
 #include <cstdio>
 #include <csignal>
 
-#ifdef VOICECAT_HAS_NET
-
 #define ASIO_STANDALONE 1
 #include <asio.hpp>
 #include <asio/signal_set.hpp>
@@ -224,21 +222,3 @@ void Server::stop() {
 }
 
 }  // namespace voicecat::server
-
-#else  // !VOICECAT_HAS_NET
-
-namespace voicecat::server {
-
-int Server::run() {
-    std::fprintf(stderr, "[server] stub: VOICECAT_HAS_NET not defined (build with dev preset)\n");
-    std::printf("  server_name : %s\n", cfg_.server_name.c_str());
-    std::printf("  data_dir    : %s\n", cfg_.data_dir.c_str());
-    std::printf("  bind_port   : %u\n", cfg_.bind_port);
-    return 0;
-}
-
-void Server::stop() {}
-
-}  // namespace voicecat::server
-
-#endif  // VOICECAT_HAS_NET