refactor(ui): scaffold ModelState and ViewState types (improvise-3vr)

Step 1 of vb4 (Split AppState into ModelState + ViewState): introduce the
named slice types in src/ui/app.rs as empty structs, with doc comments
pointing at the follow-up issues that fill them in. App still owns every
field directly; subsequent steps migrate the fields.

A structural test locks in that both types are constructible.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/ui/app.rs

@@ -185,6 +185,19 @@ impl AppMode {
     }
 }
 
+/// Document state slice: the workbook and its IO bookkeeping. Distinct from
+/// `Workbook` itself (which is pure document semantics in `improvise-core`)
+/// because `file_path` and `dirty` are persistence-layer concerns. Filled in
+/// by improvise-x2c (vb4 step 2).
+#[derive(Debug, Default)]
+pub struct ModelState {}
+
+/// UI session-state slice: mode, cursors, panels, buffers, navigation stacks,
+/// and other per-session state that does not persist to disk. Filled in by
+/// improvise-ew0 (vb4 step 3).
+#[derive(Debug, Default)]
+pub struct ViewState {}
+
 pub struct App {
     pub workbook: Workbook,
     pub file_path: Option<PathBuf>,
@@ -472,6 +485,15 @@ impl App {
 mod tests {
     use super::*;
 
+    /// improvise-3vr: ModelState and ViewState are the named slices of App
+    /// state introduced by the vb4 refactor. Step 1 only requires that the
+    /// types exist and are constructible; subsequent steps move fields in.
+    #[test]
+    fn model_state_and_view_state_are_constructible() {
+        let _: ModelState = ModelState::default();
+        let _: ViewState = ViewState::default();
+    }
+
     fn two_col_model() -> App {
         let mut wb = Workbook::new("T");
         wb.add_category("Row").unwrap(); // → Row axis