Expressions cannot read across a link #85

${x} resolves only to elements of the same form, so a condition cannot read a value from a linked row. Drafting the ROPA schema (#65) hit this twice, and the consequence is a correctness problem rather than an ergonomic one:

• A processing record re-asks "is this special category data?" when the linked data_fields row already records it.
• It re-asks "is this outside the UK/EEA?" when the linked organisations row already implies it.

The register therefore stores the same fact twice and nothing stops the copies disagreeing. In a compliance record that is exactly the failure mode that destroys credibility.

The same capability solves a second problem: select_one cannot filter its source table. Three places want it — data_fields.product should offer only product-level scopes, the justification dropdowns only justifications of the matching kind, and agreement only agreements with the chosen recipient. Without it a user can pick a combination that is not merely wrong but incoherent.

Sketch: ${data_field.special_category} in the expression language, plus a filter: on a select_one's parameters written in the same language.

Acceptance: a condition can read a field of a linked row; a select_one can restrict its options by an expression over the current form; the ROPA schema stops duplicating facts it can reach by link.

Found while drafting the ROPA schema (#65) and building provenance (#82).