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/include/voicecat.h

@@ -8,13 +8,9 @@
  * Design: docs/architecture.md §4. Everything here is async + event-driven — calls return
  * immediately and results/state changes arrive via the vc_callbacks.on_event callback.
  *
- * STATUS: real, behind VOICECAT_HAS_NET (the `dev`/`release`/`server-release` presets —
- * vcpkg deps on; see docs/building.md). As of M3, control plane, voice, multi-stream, device
- * enumeration, VAD/PTT, and stereo playback all work for real via core/src/core/client.cpp.
- * The no-deps `skeleton` preset still links a stub vc_client that returns VC_ERR_NOT_IMPLEMENTED
- * for everything below `connect`, purely to keep that skeleton build green. webrtc AEC/NS/AGC
- * remains an inert passthrough regardless of preset (no Windows/MSVC port upstream —
- * docs/voice.md §8/§11, PROGRESS.md).
+ * STATUS: real. Control plane, voice, multi-stream, device enumeration, VAD/PTT, and stereo
+ * playback all work via core/src/core/client.cpp. webrtc AEC/NS/AGC remains an inert passthrough
+ * (no Windows/MSVC port upstream — docs/voice.md §8/§11, PROGRESS.md).
  */
 #ifndef VOICECAT_H
 #define VOICECAT_H
@@ -56,7 +52,7 @@ extern "C" {
 /* ── Result codes ─────────────────────────────────────────────────────────── */
 typedef enum vc_result {
     VC_OK = 0,
-    VC_ERR_NOT_IMPLEMENTED = 1, /* skeleton stub */
+    VC_ERR_NOT_IMPLEMENTED = 1,
     VC_ERR_INVALID_ARG = 2,
     VC_ERR_NOT_CONNECTED = 3,
     VC_ERR_ALREADY = 4,