Implements draft-fossil-submanifests-00 section 6: zbits over the decoded filename, the group scan with its MIN/MAX member bounds, and the final-group merge. One scan serves height 0 and every height above it, so the merge applies uniformly rather than through two code paths that can drift; appendix A.4 is the vector that fails when they do.

Construction emits filenames through a minimal encoder scoped to its own output only. Existing emission keeps the escaping it has always used, so no check-in already written changes its bytes. R cards are emitted for tree manifests alone.

Nothing emits yet: the construction is wired behind a disabled flag, leaves stored before the nodes that reference them. The gate that turns it on is a later change. Tests pin k = 3, the value the appendix vectors use, and cover byte-identity against A.2, A.3 and A.4, determinism across repeated builds, and a constructed tree walking back to the entries it partitioned. check-in: 9d7992613f user: dmestas tags: sub-manifest