fix(import): a header-only or measureless file keeps its identity and a view of its own (improvise-lpzu, improvise-8jk6.13, improvise-8jk6.12)

ImportPipeline::from_merge seeds the File items and every file's columns
from Merged::sources, so a header-only CSV keeps its File item and its
columns (unseen columns are proposed as Labels, as an all-null column
is). A file with no measure column gets a records view filtered to it
instead of paging onto another file's hidden measure. Four per-file-view
rules the review found untested now each have a test that fails when
the rule is broken.

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

tests/folder_import_views.rs

@@ -19,15 +19,16 @@ fn reloaded_folder_import() -> App {
             "maat/coupling.csv",
             "entity,coupled,degree,average-revs\nsrc/a.rs,src/b.rs,80,5\nsrc/b.rs,src/c.rs,40,3\n",
         ),
+        ("maat/soc.csv", "entity,soc\n"),
     ];
     for (path, body) in files {
         fs.write(std::path::Path::new(path), body.as_bytes())
             .unwrap();
     }
     let merged = merge_csvs_from(&fs, &files.map(|(p, _)| p)).unwrap();
-    let mut pipeline = ImportPipeline::new(serde_json::Value::Array(merged.records));
-    pipeline.sources = merged.sources;
-    let (wb, _) = pipeline.build_model_with_warnings().unwrap();
+    let (wb, _) = ImportPipeline::from_merge(merged)
+        .build_model_with_warnings()
+        .unwrap();
     App::new(parse_md(&format_md(&wb)).unwrap(), None)
 }
 
@@ -54,4 +55,11 @@ fn a_saved_per_file_view_projects_only_its_file() {
             .collect();
         assert_eq!(drawn, texts, "{view}");
     }
+
+    // A header-only file: its item exists, and its view has nothing editable.
+    let soc = grid_in(&app, "soc", whole);
+    let file = soc.page_coords.iter().find(|p| p.cat == "File").unwrap();
+    assert_eq!(file.selected, "soc");
+    assert!(soc.records, "a file with no measure opens in records mode");
+    assert_eq!(soc.extent.rows, 0, "{soc:?}");
 }