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/session_registry.cpp

@@ -1,7 +1,5 @@
 #include "session_registry.h"
 
-#ifdef VOICECAT_HAS_NET
-
 #include <atomic>
 #include <chrono>
 #include <mutex>
@@ -554,5 +552,3 @@ std::optional<voicecat::v1::AudioConfig> SessionRegistry::channel_audio_config(
 }
 
 }  // namespace voicecat::server
-
-#endif  // VOICECAT_HAS_NET