Make CosyVoice the default macOS engine

[ivan-digital]

Summary

• make CosyVoice 3 the default macOS engine preference while keeping Windows/Linux fallback on VoxCPM2
• align the Swift sidecar process-level fallback with the Studio default
• update English/Chinese docs and repo notes for the new default
• add a unit test covering the default engine

Notes

• CosyVoice still requires a reference transcript; this PR only changes the default engine selection and docs.

Test plan

• pnpm test
• pnpm tsc --noEmit
• pnpm build
• cargo test --manifest-path src-tauri/Cargo.toml --lib
• cd swift-sidecar && swift build