POC 1 (core offset computation) and POC 2 (russh-sftp round-trip)
are complete with 26 passing tests. Key architectural finding:
two layout modes are needed — packed sequential for protocol wire
formats (LayoutBuilder/SequentialReader) and aligned static for
mmap-friendly formats (OffsetMap).
The jsonschema crate's custom keyword API handles all 16 TypeDef:*
kinds. TUnion byte-offset discriminator dispatch confirmed against
russh-sftp's own serialization byte-for-byte.