The serving primitive for the forthcoming flat-MP4 VOD path: present a synthesized faststart MP4 header followed by a byte-range window of a canonical MUXL blob as a single blob.Reader, so it drops straight into the existing Range/serveBlobRange machinery. muxl's flat-header synthesis owns all absolute offsets (the moov's co64 already points into the [header][body] address space), so we serve exactly header ++ body[off:off+len] — the whole blob for a full VOD, a contiguous sub-range for a clip. No muxl dependency yet; this is the load-bearing read primitive the playback endpoint will wrap once muxl ships the metafile -> flat-header synth. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>