Prerequisite tables

Terminology and form scaffolding that no source model describes.

Four tables are not derived from any source specification, because neither openEHR nor FHIR describes them. They are the scaffolding everything else hangs on.

Table	What it holds
`valueset`	One row per coded field in the schema.
`valueset_code`	The codes a value set admits, with the display text that must accompany them.
`form`	The clinical spine — every entry belongs to a form.
`form_block`	Ordered blocks within a form.

attachment used to live here too. It is now compiled like any other table, from the FHIR Attachment datatype merged with CLUSTER.media_file.v1 — see attachment.