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/net/transport.h

@@ -4,9 +4,8 @@
  * Design: docs/architecture.md (Net thread), docs/protocol.md §1 (framing).
  * Implementation uses standalone Asio for sockets and timers.
  *
- * The real classes are compiled only when VOICECAT_HAS_NET is defined (dev/release/
- * server-release — vcpkg deps on). The skeleton-preset stub definitions below keep the
- * no-deps build green.
+ * Design: docs/architecture.md (Net thread), docs/protocol.md §1 (framing).
+ * Implementation uses standalone Asio for sockets and timers.
  */
 #ifndef VOICECAT_NET_TRANSPORT_H
 #define VOICECAT_NET_TRANSPORT_H
@@ -14,8 +13,6 @@
 #include <cstdint>
 #include <string>
 
-#ifdef VOICECAT_HAS_NET
-
 #define ASIO_STANDALONE 1
 #include <asio.hpp>
 
@@ -226,21 +223,4 @@ class UdpMediaChannel {
 
 }  // namespace voicecat::net
 
-#else  // !VOICECAT_HAS_NET — skeleton stubs for the dev preset
-
-namespace voicecat::net {
-
-class TcpControlChannel {
-   public:
-    bool connected() const { return false; }
-};
-
-class UdpMediaChannel {
-   public:
-    bool bound() const { return false; }
-};
-
-}  // namespace voicecat::net
-
-#endif  // VOICECAT_HAS_NET
 #endif  // VOICECAT_NET_TRANSPORT_H