Preview an import #39

Outcome. A collector uploads an export file and sees what applying it would do, with an error per row that can't be applied. Nothing is written.

• Validates the format version and resolves external ids against the local catalog.
• The first upload in the app: ADR 0013's Consequences say the first upload feature owes upload validation.

A candidate from the roadmap. It is refined into a spec when its turn comes, and checked then against the north star's hosted-door rules and spec rules (one outcome, one PR; a spec that adds collector-owned data extends the export in the same PR).