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>

tests/CMakeLists.txt

@@ -6,7 +6,6 @@ target_link_libraries(test_smoke PRIVATE voicecat::voicecat)
 target_compile_features(test_smoke PRIVATE cxx_std_20)
 add_test(NAME smoke COMMAND test_smoke)
 
-# frame_codec has no third-party deps; runs under both skeleton and dev.
 # Needs core/src on the include path to reach internal headers (protocol/, session/, etc.).
 add_executable(test_frame_codec test_frame_codec.cpp)
 target_link_libraries(test_frame_codec PRIVATE voicecat::voicecat)
@@ -14,8 +13,7 @@ target_compile_features(test_frame_codec PRIVATE cxx_std_20)
 target_include_directories(test_frame_codec PRIVATE ${CMAKE_SOURCE_DIR}/core/src)
 add_test(NAME frame_codec COMMAND test_frame_codec)
 
-if(VOICECAT_USE_VCPKG_DEPS)
-    set(VC_TEST_INTERNAL_INCLUDES
+set(VC_TEST_INTERNAL_INCLUDES
         ${CMAKE_SOURCE_DIR}/core/src
         ${CMAKE_SOURCE_DIR}/server/src
         ${CMAKE_BINARY_DIR}/core/generated)  # protobuf-generated headers
@@ -247,4 +245,3 @@ if(VOICECAT_USE_VCPKG_DEPS)
     target_include_directories(test_recv_noise_reduction PRIVATE ${VC_TEST_INTERNAL_INCLUDES})
     add_test(NAME recv_noise_reduction COMMAND test_recv_noise_reduction)
     set_tests_properties(recv_noise_reduction PROPERTIES TIMEOUT 90)
-endif()