Wires the vat_code snapshot (migration 130) through the write paths: quote
create/update, the main invoice create, and the Storno carry-over (so a
cancellation exports the same code as the invoice it reverses). Guarded with
hasColumnCached; reads payload.vatCode (sent by the editor dropdown, coming in a
later slice — inert until then, falls back to the rate→code map). 72 tests pass.
Luca
5b52969e36