build(cmake): clean up presets, add release/apple presets, cross-platform triplets

Rationalize the preset set to match the project's actual state (past M5):
- Rename dev->skeleton (no-deps stub smoke), m1-dev->dev (default dev preset)
- Drop m2-dev (cache-identical to m1-dev)
- Add release preset (optimized + tests on, symbols kept)
- Strip server-release binaries (-s linker flag)
- Add apple-dev/apple-ios/apple-ios-sim scaffolding presets for XCFramework

Add cmake/voicecat-toolchain.cmake wrapper that auto-resolves the vcpkg
triplet from the host platform (x64-mingw-static/x64-linux/arm64-osx) so
the main presets work on Windows/Linux/macOS without per-OS variants.

Update all docs (building.md, CLAUDE.md, README.md, AGENTS.md, deployment.md,
tech-stack.md, client READMEs) and stale preset-name references in code
comments. No C++ behavior changes — the core was already portable.

tests/test_smoke.cpp

@@ -43,7 +43,7 @@ int main() {
     CHECK(vc_connect(c, nullptr, 1) == VC_ERR_INVALID_ARG);
     CHECK(vc_send_text(c, VC_TEXT_CHANNEL, 0, nullptr) == VC_ERR_INVALID_ARG);
 
-    // Under dev preset: NOT_IMPLEMENTED. Under m1-dev: VC_OK (async connect).
+    // Under skeleton preset: NOT_IMPLEMENTED. Under dev: VC_OK (async connect).
     vc_result rc_connect = vc_connect(c, "127.0.0.1", 8384);
     CHECK(rc_connect == VC_ERR_NOT_IMPLEMENTED || rc_connect == VC_OK);