refactor(ui): move UI session fields into ViewState (improvise-ew0)

Step 3 of vb4. Populates ViewState with the 20 UI session fields (mode,
status_msg, wizard, search_query, search_mode, three panel-open flags,
three panel cursors, formula_cursor, yanked, tile_cat_idx, two view nav
stacks, drill_state, help_page, expanded_cats, buffers, transient_keymap)
and routes every read/write site through app.view_state.X. App now
contains only model_state, view_state, and the runtime/derived residue
(term dims, layout, last_autosave, abort_effects, keymap_set).

ViewState gets a manual Default impl mirroring the previous App::new
field initialisers; AppMode has no Default of its own so AppMode::Normal
is the explicit baseline. Effect::apply still takes &mut App; narrowing
remains step 5 (improvise-drg).

A structural test (app_view_state_owns_ui_session_fields) locks in the
20-field layout. 624 tests pass workspace-wide (+1 new). cargo clippy
--workspace --tests clean.

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

src/draw.rs

@@ -61,7 +61,7 @@ pub fn run_tui(
     if let Some(json) = import_value {
         app.start_import_wizard(json);
     } else if app.is_empty_model() {
-        app.mode = AppMode::Help;
+        app.view_state.mode = AppMode::Help;
     }
 
     loop {
@@ -82,7 +82,7 @@ pub fn run_tui(
 
         app.autosave_if_needed();
 
-        if matches!(app.mode, AppMode::Quit) {
+        if matches!(app.view_state.mode, AppMode::Quit) {
             break;
         }
     }
@@ -167,21 +167,21 @@ fn draw(f: &mut Frame, app: &App) {
     draw_bottom_bar(f, main_chunks[3], app);
 
     // Overlays (rendered last so they appear on top)
-    if matches!(app.mode, AppMode::Help) {
-        f.render_widget(HelpWidget::new(app.help_page), size);
+    if matches!(app.view_state.mode, AppMode::Help) {
+        f.render_widget(HelpWidget::new(app.view_state.help_page), size);
     }
-    if matches!(app.mode, AppMode::ImportWizard)
-        && let Some(wizard) = &app.wizard
+    if matches!(app.view_state.mode, AppMode::ImportWizard)
+        && let Some(wizard) = &app.view_state.wizard
     {
         f.render_widget(ImportWizardWidget::new(wizard), size);
     }
     // ExportPrompt now uses the minibuffer at the bottom bar.
-    if app.is_empty_model() && matches!(app.mode, AppMode::Normal | AppMode::CommandMode { .. }) {
+    if app.is_empty_model() && matches!(app.view_state.mode, AppMode::Normal | AppMode::CommandMode { .. }) {
         draw_welcome(f, main_chunks[1]);
     }
 
     // Which-key popup: show available completions after a prefix key
-    if let Some(ref km) = app.transient_keymap {
+    if let Some(ref km) = app.view_state.transient_keymap {
         let hints = km.binding_hints();
         f.render_widget(WhichKeyWidget::new(&hints), size);
     }
@@ -215,7 +215,7 @@ fn draw_title(f: &mut Frame, area: Rect, app: &App) {
 }
 
 fn draw_content(f: &mut Frame, area: Rect, app: &App) {
-    let side_open = app.formula_panel_open || app.category_panel_open || app.view_panel_open;
+    let side_open = app.view_state.formula_panel_open || app.view_state.category_panel_open || app.view_state.view_panel_open;
 
     let grid_area;
     if side_open {
@@ -229,9 +229,9 @@ fn draw_content(f: &mut Frame, area: Rect, app: &App) {
 
         let side = chunks[1];
         let panel_count = [
-            app.formula_panel_open,
-            app.category_panel_open,
-            app.view_panel_open,
+            app.view_state.formula_panel_open,
+            app.view_state.category_panel_open,
+            app.view_state.view_panel_open,
         ]
         .iter()
         .filter(|&&b| b)
@@ -239,26 +239,26 @@ fn draw_content(f: &mut Frame, area: Rect, app: &App) {
         let ph = side.height / panel_count.max(1);
         let mut y = side.y;
 
-        if app.formula_panel_open {
+        if app.view_state.formula_panel_open {
             let a = Rect::new(side.x, y, side.width, ph);
-            let content = FormulaContent::new(&app.model_state.workbook.model, &app.mode);
-            f.render_widget(Panel::new(content, &app.mode, app.formula_cursor), a);
+            let content = FormulaContent::new(&app.model_state.workbook.model, &app.view_state.mode);
+            f.render_widget(Panel::new(content, &app.view_state.mode, app.view_state.formula_cursor), a);
             y += ph;
         }
-        if app.category_panel_open {
+        if app.view_state.category_panel_open {
             let a = Rect::new(side.x, y, side.width, ph);
             let content = CategoryContent::new(
                 &app.model_state.workbook.model,
                 app.model_state.workbook.active_view(),
-                &app.expanded_cats,
+                &app.view_state.expanded_cats,
             );
-            f.render_widget(Panel::new(content, &app.mode, app.cat_panel_cursor), a);
+            f.render_widget(Panel::new(content, &app.view_state.mode, app.view_state.cat_panel_cursor), a);
             y += ph;
         }
-        if app.view_panel_open {
+        if app.view_state.view_panel_open {
             let a = Rect::new(side.x, y, side.width, ph);
             let content = ViewContent::new(&app.model_state.workbook);
-            f.render_widget(Panel::new(content, &app.mode, app.view_panel_cursor), a);
+            f.render_widget(Panel::new(content, &app.view_state.mode, app.view_state.view_panel_cursor), a);
         }
     } else {
         grid_area = area;
@@ -270,10 +270,10 @@ fn draw_content(f: &mut Frame, area: Rect, app: &App) {
             app.model_state.workbook.active_view(),
             &app.model_state.workbook.active_view,
             &app.layout,
-            &app.mode,
-            &app.search_query,
-            &app.buffers,
-            app.drill_state.as_ref(),
+            &app.view_state.mode,
+            &app.view_state.search_query,
+            &app.view_state.buffers,
+            app.view_state.drill_state.as_ref(),
         ),
         grid_area,
     );
@@ -284,16 +284,17 @@ fn draw_tile_bar(f: &mut Frame, area: Rect, app: &App) {
         TileBar::new(
             &app.model_state.workbook.model,
             app.model_state.workbook.active_view(),
-            &app.mode,
-            app.tile_cat_idx,
+            &app.view_state.mode,
+            app.view_state.tile_cat_idx,
         ),
         area,
     );
 }
 
 fn draw_bottom_bar(f: &mut Frame, area: Rect, app: &App) {
-    if let Some(mb) = app.mode.minibuffer() {
+    if let Some(mb) = app.view_state.mode.minibuffer() {
         let buf = app
+            .view_state
             .buffers
             .get(mb.buffer_key)
             .map(|s| s.as_str())
@@ -314,25 +315,25 @@ fn draw_bottom_bar(f: &mut Frame, area: Rect, app: &App) {
 }
 
 fn draw_status(f: &mut Frame, area: Rect, app: &App) {
-    let search_part = if app.search_mode {
-        format!("  /{}▌", app.search_query)
+    let search_part = if app.view_state.search_mode {
+        format!("  /{}▌", app.view_state.search_query)
     } else {
         String::new()
     };
 
-    let msg = if !app.status_msg.is_empty() {
-        app.status_msg.as_str()
+    let msg = if !app.view_state.status_msg.is_empty() {
+        app.view_state.status_msg.as_str()
     } else {
         app.hint_text()
     };
 
-    let yank_indicator = if app.yanked.is_some() { " [yank]" } else { "" };
+    let yank_indicator = if app.view_state.yanked.is_some() { " [yank]" } else { "" };
     let view_badge = format!("  {}{}  ", app.model_state.workbook.active_view, yank_indicator);
 
-    let left = format!("  {}{search_part}  {msg}", mode_name(&app.mode));
+    let left = format!("  {}{search_part}  {msg}", mode_name(&app.view_state.mode));
     let line = fill_line(left, &view_badge, area.width);
 
-    f.render_widget(Paragraph::new(line).style(mode_style(&app.mode)), area);
+    f.render_widget(Paragraph::new(line).style(mode_style(&app.view_state.mode)), area);
 }
 
 fn draw_welcome(f: &mut Frame, area: Rect) {