Document table structure of fluo table

The Fluo table has a rather complex structure of prefix masking in the timestamp of the key as well as byte arrays in the value that mean different things based on the first byte and a timestamp in the rest of the bytes.

It would be useful if we could document the working model somewhere so that it's easy for a user to visualize.

No file, test, or entry point is named in the issue. Start by locating the Fluo table schema and the code that encodes its key and value bytes, then document the prefix masking, first-byte meanings, and timestamp layout in the project's appropriate documentation location. Done means users can visualize the table structure without reading the implementation.

Written by the indexing model from the issue text.