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/test_m2_voice.cpp

@@ -14,8 +14,6 @@
 #include <cstdio>
 #include <cstring>
 
-#ifdef VOICECAT_HAS_NET
-
 #include <atomic>
 #include <chrono>
 #include <condition_variable>
@@ -53,9 +51,7 @@
 #include "server.h"
 #include "db.h"
 
-#ifdef VOICECAT_HAS_OPUS
 #include "codec/opus_codec.h"
-#endif
 
 using namespace voicecat;
 using namespace voicecat::net;
@@ -441,7 +437,6 @@ int main() {
     constexpr int kFramesToSend = 50;
     constexpr int kFrameSamples = 960;  // 20 ms @48 kHz
 
-#ifdef VOICECAT_HAS_OPUS
     voicecat::codec::OpusEncoder enc;
     {
         voicecat::codec::OpusParams p;
@@ -450,24 +445,17 @@ int main() {
         p.fec         = true;
         CHECK(enc.init(p));
     }
-#endif
 
     std::vector<uint8_t> aead_buf(4096);  // scratch
 
     for (int i = 0; i < kFramesToSend; ++i) {
         auto pcm = make_sine_frame(i, kFrameSamples);
 
-#ifdef VOICECAT_HAS_OPUS
         uint8_t opus_buf[1000];
         int opus_len = enc.encode(pcm.data(), kFrameSamples, opus_buf, sizeof(opus_buf));
         if (opus_len <= 0) continue;
         const uint8_t* payload = opus_buf;
         size_t         payload_len = static_cast<size_t>(opus_len);
-#else
-        // Fallback: use raw PCM as synthetic payload
-        const uint8_t* payload = reinterpret_cast<const uint8_t*>(pcm.data());
-        size_t         payload_len = pcm.size() * sizeof(int16_t);
-#endif
 
         // Build the voice frame header (AAD).
         VoiceFrame hdr;
@@ -494,9 +482,7 @@ int main() {
         std::this_thread::sleep_for(std::chrono::milliseconds(20));
     }
 
-#ifdef VOICECAT_HAS_OPUS
     enc.destroy();
-#endif
 
     // ── B collects received frames (2s window after last send) ────────────────
     int recv_count = 0, decrypt_ok = 0;
@@ -541,12 +527,3 @@ int main() {
     std::printf("m2_voice: %d failure(s)\n", g_failures);
     return 1;
 }
-
-#else  // !VOICECAT_HAS_NET
-
-int main() {
-    std::printf("m2_voice: SKIP (VOICECAT_HAS_NET not defined)\n");
-    return 0;
-}
-
-#endif  // VOICECAT_HAS_NET