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.

core/include/voicecat.h

@@ -8,10 +8,10 @@
  * 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 `m1-dev`/`server-release` presets — vcpkg deps
- * on; see docs/building.md). As of M3, control plane, voice, multi-stream, device
+ * 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 `dev` preset still links a stub vc_client that returns VC_ERR_NOT_IMPLEMENTED
+ * 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).