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>

CMakeLists.txt

@@ -14,11 +14,6 @@ if(CMAKE_SYSTEM_NAME STREQUAL "iOS")
 endif()
 
 # ── Options ───────────────────────────────────────────────────────────────────
-# The M0 skeleton compiles with NO third-party dependencies: every subsystem is a
-# stub that returns VC_ERR_NOT_IMPLEMENTED. As each subsystem is built out, flip
-# VOICECAT_USE_VCPKG_DEPS=ON so CMake pulls the real libraries (mbedTLS, libsodium,
-# opus, protobuf, ...) via the vcpkg toolchain (see vcpkg.json / docs/tech-stack.md).
-option(VOICECAT_USE_VCPKG_DEPS "Link real third-party deps via vcpkg" OFF)
 option(VOICECAT_BUILD_SERVER   "Build voicecat-server"                ON)
 option(VOICECAT_BUILD_TOOLS    "Build the vccli headless test client" ON)
 option(VOICECAT_BUILD_TESTS    "Build tests"                          ON)
@@ -60,9 +55,7 @@ endif()
 
 if(VOICECAT_BUILD_TOOLS)
     add_subdirectory(tools/vccli)
-    if(VOICECAT_USE_VCPKG_DEPS)
-        add_subdirectory(tools/voicecat-admin)
-    endif()
+    add_subdirectory(tools/voicecat-admin)
 endif()
 
 if(VOICECAT_BUILD_TESTS)
@@ -71,5 +64,4 @@ if(VOICECAT_BUILD_TESTS)
 endif()
 
 message(STATUS "VoiceCat ${PROJECT_VERSION} configured "
-        "(vcpkg deps: ${VOICECAT_USE_VCPKG_DEPS}, "
-        "server: ${VOICECAT_BUILD_SERVER}, tools: ${VOICECAT_BUILD_TOOLS})")
+        "(server: ${VOICECAT_BUILD_SERVER}, tools: ${VOICECAT_BUILD_TOOLS})")