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>

CMakePresets.json

@@ -12,17 +12,6 @@
         "VOICECAT_USE_VCPKG_DEPS": "ON"
       }
     },
-    {
-      "name": "skeleton",
-      "displayName": "Skeleton (no third-party deps)",
-      "description": "Builds the stub skeleton with just a C++20 compiler — no vcpkg needed. Subsystems return VC_ERR_NOT_IMPLEMENTED. Good for 'does the repo even build' smoke checks. Runs 2 tests (smoke + frame_codec).",
-      "generator": "Ninja",
-      "binaryDir": "${sourceDir}/build/skeleton",
-      "cacheVariables": {
-        "CMAKE_BUILD_TYPE": "Debug",
-        "VOICECAT_USE_VCPKG_DEPS": "OFF"
-      }
-    },
     {
       "name": "dev",
       "displayName": "Dev (full real-deps build, vcpkg)",
@@ -130,7 +119,6 @@
     }
   ],
   "buildPresets": [
-    { "name": "skeleton",       "configurePreset": "skeleton" },
     { "name": "dev",            "configurePreset": "dev" },
     { "name": "release",        "configurePreset": "release" },
     { "name": "server-release", "configurePreset": "server-release" },
@@ -140,7 +128,6 @@
     { "name": "apple-ios-sim",  "configurePreset": "apple-ios-sim" }
   ],
   "testPresets": [
-    { "name": "skeleton", "configurePreset": "skeleton", "output": { "outputOnFailure": true } },
     { "name": "dev",      "configurePreset": "dev",      "output": { "outputOnFailure": true } },
     { "name": "release",  "configurePreset": "release",  "output": { "outputOnFailure": true } }
   ]