Schemas for the file inputs that have none #388

Background

Three inputs are read with no schema defining them at all: the CSV a converter consumes, the
i18ntext/UI.*.json string tables, and the bulk-decide import file.
ADR 021's
rule 4 makes the absence of a schema the work rather than an exemption — including for CSV, which has
no official schema language and therefore needs a contract this project defines.

Depends on #384's sub-issue 1, which supplies the entry point these validate through. Independent of
sub-issue 3.

What needs to be done

1. A defined contract for the CSV converter's input — column set, which columns are required, and how
   a violation is expressed — plus its validator, since no official one exists to borrow.
2. A JSON schema for i18ntext/UI.*.json.
3. A JSON schema for the bulk-decide import file.
4. All three bound to their readers and validated through the entry point.

Expected tests

Definition of done

• All expected tests listed above start red before implementation
• All requirements implemented
• All expected tests pass (green)
• No regression in related tests
• Findings summarised in a closing comment