Changes to Fetch a content-addressed MUXL playback blob by CID. Serves either a primary fMP4 archive or a per-track init segment — both are CID-addressed and indistinguishable from the endpoint's point of view. HTTP `Range` headers are honored; HLS media playlists generated by getVideoPlaylist reference this endpoint with `EXT-X-BYTERANGE` to serve segments out of the primary blob. The `did` identifies the owning account for egress accounting: the blob itself is content-addressed and owner-agnostic, so the caller carries the attribution.