Edit a proposed model before building it

Change the period axis or an assumption value on a draft and get the re-planned model back — re-resolved against the chart of accounts and re-evaluated, so the effect of the edit is visible before anything is built.

This exists so the common edits — quarterly not monthly, the fee is 55 basis points — do not have to go back through the language model, which would re-emit every row and is free to drift a formula nobody asked about. Structural changes still belong in chat.

assumptions is merged into the draft; sending one key does not delete the others. The edit keeps the same draft_id and renews its expiry. An edit that would make the model invalid — dropping an assumption a row still references, or an axis wider than the column cap — is rejected here with the reason, not at build time.