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_frame_codec.cpp

@@ -1,7 +1,7 @@
 /*
  * test_frame_codec — unit test for FrameCodec::feed / ::emit.
  *
- * No third-party dependencies; runs under both the dev and m1-dev presets.
+ * No third-party dependencies; runs under both the skeleton and dev presets.
  * Tests: empty payload, single byte, 64 KiB, exact max-size, oversized (should reject),
  * split delivery (bytes fed one-at-a-time), and batched multi-frame delivery.
  */