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>

core/CMakeLists.txt

@@ -42,8 +42,7 @@ set_target_properties(voicecat PROPERTIES
     CXX_VISIBILITY_PRESET hidden
     VISIBILITY_INLINES_HIDDEN ON)
 
-if(VOICECAT_USE_VCPKG_DEPS)
-    find_package(protobuf CONFIG REQUIRED)
+find_package(protobuf CONFIG REQUIRED)
     find_package(unofficial-sodium CONFIG REQUIRED)
     find_package(MbedTLS CONFIG REQUIRED)
     find_package(asio CONFIG REQUIRED)
@@ -129,4 +128,3 @@ if(VOICECAT_USE_VCPKG_DEPS)
 
     # Signal to C++ code that the real networking/crypto stack is available.
     target_compile_definitions(voicecat PUBLIC VOICECAT_HAS_NET)
-endif()