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_vad_ptt_devices.cpp

@@ -18,8 +18,6 @@
  */
 #include <cstdio>
 
-#ifdef VOICECAT_HAS_NET
-
 #include <atomic>
 #include <chrono>
 #include <cmath>
@@ -35,12 +33,8 @@
 #include "server.h"
 #include "db.h"
 
-#ifdef VOICECAT_HAS_AUDIO
 #include "audio/audio_engine.h"
-#endif
-#ifdef VOICECAT_HAS_OPUS
 #include "codec/opus_codec.h"
-#endif
 
 // ── Event tracking (same shape as test_m3_multistream.cpp) ──────────────────────
 
@@ -159,7 +153,6 @@ static void test_device_enumeration() {
     for (vc_device_kind kind : {VC_DEVICE_INPUT, VC_DEVICE_OUTPUT}) {
         vc_device_list dl{};
         vc_result      r = vc_list_devices(c, kind, &dl);
-#ifdef VOICECAT_HAS_AUDIO
         CHECK(r == VC_OK);
         // Headless CI build agents may legitimately report zero devices — never assert
         // count > 0, only that the call itself succeeded and the list is well-formed.
@@ -167,9 +160,6 @@ static void test_device_enumeration() {
             CHECK(dl.items[i].id != nullptr);
             CHECK(dl.items[i].name != nullptr);
         }
-#else
-        CHECK(r == VC_ERR_NOT_IMPLEMENTED);
-#endif
         vc_free_device_list(&dl);
         vc_free_device_list(&dl);  // idempotent — must not crash on a second call
     }
@@ -179,7 +169,6 @@ static void test_device_enumeration() {
 }
 
 // ── 4. Stereo playback mixer (white-box, no audio hardware needed) ──────────────
-#if defined(VOICECAT_HAS_AUDIO) && defined(VOICECAT_HAS_OPUS)
 static void test_stereo_mix() {
     voicecat::audio::AudioEngine engine;
     voicecat::audio::AudioParams p;
@@ -245,7 +234,7 @@ static void test_stereo_mix() {
 // and mixes — asserting L != R across the frame. A mono-downmixed-then-upmixed bitstream would
 // have L == R. Mirrors test_stereo_mix but routes the encode side through the loopback
 // accumulator path that the fix touches (feed_loopback_for_test → on_loopback's accumulator).
-#if defined(VOICECAT_HAS_LOOPBACK) && defined(VOICECAT_HAS_OPUS)
+#if defined(VOICECAT_HAS_LOOPBACK)
 static void test_loopback_stereo_capture() {
     voicecat::audio::AudioEngine engine;
     voicecat::audio::AudioParams p;
@@ -382,14 +371,12 @@ static void test_playout_resync() {
     engine.stop();
     std::printf("test_playout_resync: ok (energy=%lld)\n", static_cast<long long>(energy));
 }
-#endif  // VOICECAT_HAS_AUDIO && VOICECAT_HAS_OPUS
 
 // ── 5. Capture-frame accumulation (white-box, no audio hardware needed) ──────────
 // Regression for the capture-side analogue of the playback ring fix: miniaudio's capture
 // callback fires at the hardware period (commonly 480 samples on WASAPI shared mode), while
 // opus_encode() requires exactly frame_samples_ (960). Sub-frame chunks must be accumulated;
 // the callback must receive exactly 960-sample frames regardless of input chunk size.
-#ifdef VOICECAT_HAS_AUDIO
 static void test_capture_frame_accumulation() {
     voicecat::audio::AudioEngine engine;
     voicecat::audio::AudioParams p;
@@ -436,7 +423,6 @@ static void test_capture_frame_accumulation() {
     engine.stop();
     std::printf("test_capture_frame_accumulation: ok (callbacks=%d)\n", call_count.load());
 }
-#endif  // VOICECAT_HAS_AUDIO
 
 // ── 2/3. VAD + PTT gate, through the real ABI against a real server ─────────────
 static void test_vad_and_ptt_gate() {
@@ -601,7 +587,6 @@ static void test_vad_and_ptt_gate() {
 // downmix). Mirrors test_loopback_stereo_capture but routes through the mic capture
 // accumulator (feed_capture_for_test with channels=2) instead of the loopback path.
 // This is the headless CI test for the iOS stereo built-in mic feature (Part D).
-#if defined(VOICECAT_HAS_AUDIO) && defined(VOICECAT_HAS_OPUS)
 static void test_stereo_mic_capture() {
     voicecat::audio::AudioEngine engine;
     voicecat::audio::AudioParams p;
@@ -674,7 +659,6 @@ static void test_stereo_mic_capture() {
     std::printf("test_stereo_mic_capture: ok (total_diff=%lld, seen_channels=%d)\n",
                 static_cast<long long>(total_diff), seen_channels);
 }
-#endif
 
 // ── 6. Stereo mic capture on a MONO channel (downmix safety) ──────────────────
 // A stereo mic (vc_set_capture_channels=2) can be enabled while on a mono channel. The mic
@@ -683,7 +667,6 @@ static void test_stereo_mic_capture() {
 // opus_encode makes it read 2× the samples it should (wrong pitch / garbage). This mirrors that
 // fold and proves the result is a valid mono bitstream that decodes to the expected averaged
 // signal, rather than half-length junk.
-#if defined(VOICECAT_HAS_AUDIO) && defined(VOICECAT_HAS_OPUS)
 static void test_stereo_mic_mono_channel() {
     voicecat::codec::OpusParams mono_params;
     mono_params.stereo      = false;  // mono channel — encoder is mono
@@ -727,11 +710,9 @@ static void test_stereo_mic_mono_channel() {
     std::printf("test_stereo_mic_mono_channel: ok (opus_len=%d, energy=%lld)\n",
                 opus_len, static_cast<long long>(energy));
 }
-#endif
 
 int main() {
     test_device_enumeration();
-#if defined(VOICECAT_HAS_AUDIO) && defined(VOICECAT_HAS_OPUS)
     test_stereo_mix();
 #if defined(VOICECAT_HAS_LOOPBACK)
     test_loopback_stereo_capture();
@@ -739,10 +720,7 @@ int main() {
     test_stereo_mic_capture();
     test_stereo_mic_mono_channel();
     test_playout_resync();
-#endif
-#ifdef VOICECAT_HAS_AUDIO
     test_capture_frame_accumulation();
-#endif
     test_vad_and_ptt_gate();
 
     if (g_failures == 0) {
@@ -752,12 +730,3 @@ int main() {
     std::printf("vad_ptt_devices: %d failure(s)\n", g_failures);
     return 1;
 }
-
-#else  // !VOICECAT_HAS_NET
-
-int main() {
-    std::printf("vad_ptt_devices: SKIP (VOICECAT_HAS_NET not defined)\n");
-    return 0;
-}
-
-#endif  // VOICECAT_HAS_NET