Unified encoding and decoding for ReifyDB. Every byte layout that crosses a boundary lives here: the single type-tag namespace, the canonical self-describing value codec, the RBCF columnar frame format, the row storage codec, the order-preserving key codec, and the FFI cell codecs.
Invariant: encode and decode halves are colocated in this crate and share one tag scheme. A tag, layout, or width change is a coordinated workspace change; no consumer may hand-roll these bytes.