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/src/session/session.h

@@ -12,9 +12,7 @@
 #include <utility>
 #include <vector>
 
-#ifdef VOICECAT_HAS_NET
 #include "proto/voicecat.pb.h"
-#endif
 
 namespace voicecat::session {
 
@@ -88,11 +86,9 @@ class SessionModel {
     // Returns {nullptr, nullptr} if not found.
     std::pair<const User*, const Stream*> find_user_by_ssrc(uint32_t ssrc) const;
 
-#ifdef VOICECAT_HAS_NET
     void apply_snapshot(const voicecat::v1::ServerStateSnapshot& snap);
     void apply_user_event(const voicecat::v1::UserEvent& ev);
     void apply_channel_event(const voicecat::v1::ChannelEvent& ev);
-#endif
 
    private:
     std::vector<Channel> channels_;