All compressed blocks, concatenated.
blocks offsets, length blockCount + 1. Block i occupies
[index[i], index[i + 1]), so the last entry is blocks.byteLength.
One byte per block: 1 = stored verbatim, 0 = deflated. Incompressible blocks (already-compressed payloads embedded in a STEP file) would otherwise grow, and a store that can grow is not one you can gate on size.
Logical length of the original source.
The wire/consructor shape: compressed blocks plus their offset index.