feat(native-audio): add native multi-output routing

[grazzolini]

Summary

• Add native output groups for project audio, stems, and cues. Sources resolving to the same device share one stream, with existing gains preserved.
• Apply transport commands across groups and expose routing requests, effective destinations, and fallback state through Tauri IPC.
• Attempt System Default recovery for unavailable selections. Failed default recovery stops playback while preserving position and routing preferences.

Motivation

Refs #505. Layer 1 of a three-PR review stack: native audio, shared state/engine adapters, then UI/UX. The complete stack is the runnable acceptance target.

Scope is routing first: device-clock synchronization, drift correction, latency compensation, and per-output volume controls are deferred.

Testing

• Native audio tests: 233 passed (cargo test --manifest-path apps/desktop/src-tauri/Cargo.toml native_audio).
• IPC tests: 17 passed (pnpm --filter @tuneforge/desktop exec vitest run src/lib/nativeAudio.test.ts).
• cargo check, pnpm typecheck, and git diff --check passed.
• Full integration and platform validation, including Android, Linux, and hardware routing, will run against the complete stack after all three PRs are open. This includes pnpm lint, pnpm test, and node scripts/capture-release-media.mjs --run.

Checklist

• Native-layer implementation and focused review complete
• Complete-stack integration and platform validation
• Shared routing state, Web Audio, UI/UX, docs, and changelog in subsequent layers