test(import): a folder of only header-only files kept no record and refuses (improvise-8jk6.6)

After merging PR #314, a header-only file's seeded File proposal means the
category check alone no longer refuses an import that kept no record.
nothing_to_import does, and this pins it. The docs now name from_merge
(switched to from_csv in the merge) among the CSV entry points.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VdfEGXA7aJ6ftMsCc9PJze

crates/improvise-io/src/import/wizard.rs

@@ -3338,6 +3338,16 @@ mod tests {
         assert_eq!(wb.model.get_cell(&key), Some(&CellValue::Number(5.0)));
     }
 
+    /// improvise-8jk6.6 meets lpzu's header-only rule: a folder of nothing but
+    /// headers kept no record. Evidence: `context/test-evidence/formula-and-io.md`.
+    #[test]
+    fn a_folder_of_only_header_only_files_keeps_no_record_and_refuses() {
+        let p = merged_pipeline(&[("m/a.csv", "entity,x\n"), ("m/b.csv", "entity,y\n")]);
+        assert!(p.proposals.iter().any(|f| f.field == "File"), "setup");
+        let msg = p.build_model().expect_err("refuses").to_string();
+        assert_eq!(msg, "Nothing to import: the source has no records");
+    }
+
     /// improvise-8jk6.6: the wizard asks on the review step, where another
     /// path can still be chosen. Evidence: the test-evidence doc.
     #[test]