Function build_import_spec 

pub fn build_import_spec(form: &ImportForm) -> Result<ImportSpec, String>

Build an ImportSpec from the wizard’s form fields. Validates as the CLI does: a non-empty path, a numeric measure id, a non-empty measure name and value column, and at least one fully-filled dimension row (a blank spacer row — all three fields empty — is skipped, not an error). Pure: no egui, no I/O; improv_storage_csv::import_csv does the real work.