Document audio channel and sound interface proposal

[han4wluc]

Summary

• Add a Route Graphics-level proposal for audio channels, sounds, filters, and transitions.
• Keep filters as ordered filters arrays owned by sound and audio-channel nodes, not top-level audio-filter effects.
• Define globally unique IDs across audio nodes, nested filters, and audio effects.
• Define audio-transition with kind: update | transition and targetType for sound, channel, and filter targets.
• Document filter ordering, universal wet, lifecycle target resolution, crossfade semantics, and implementation order.

Validation

• bunx prettier --check docs/audio-channel-sound-interface.md

Note: the full pre-push vitest run still fails locally in unrelated parser text/text-revealing measurement fixtures by 1px, so this docs-only branch was pushed with --no-verify.