### Spec: v1.1 multi-rendition envelope (SPEC.md §6.1)

- New magic `WAI2` for envelopes that carry multiple renditions of the
  same content (e.g. EnCodec + Opus, or AVIF + JPEG-XL). Reframed from
  "graceful fallback for missing capability" to **policy selection** —
  in a self-contained ecosystem every sink has every required
  capability; renditions exist so the sink picks by compute budget,
  bandwidth, or runtime preference (browser-native vs WebGPU vs WebNN
  vs native ML). v1.0 (`WAI1`) remains valid and unchanged.

