refactor: break Model↔View cycle, introduce Workbook wrapper

Model is now pure data (categories, cells, formulas, measure_agg) with
no references to view/. The Workbook struct owns the Model together
with views and the active view name, and is responsible for cross-slice
operations (add/remove category → notify views, view management).

- New: src/workbook.rs with Workbook wrapper and cross-slice helpers
  (add_category, add_label_category, remove_category, create_view,
  switch_view, delete_view, normalize_view_state).
- Model: strip view state and view-touching methods. recompute_formulas
  remains on Model as a primitive; the view-derived none_cats list is
  gathered at each call site (App::rebuild_layout, persistence::load)
  so the view dependency is explicit, not hidden behind a wrapper.
- View: add View::none_cats() helper.
- CmdContext: add workbook and view fields so commands can reach both
  slices without threading Model + View through every call.
- App: rename `model` field to `workbook`.
- Persistence (save/load/format_md/parse_md/export_csv): take/return
  Workbook so the on-disk format carries model + views together.
- Widgets (GridWidget, TileBar, CategoryContent, ViewContent): take
  explicit &Model + &View instead of routing through Model.

Tests updated throughout to reflect the new shape. View-management
tests that previously lived on Model continue to cover the same
behaviour via a build_workbook() helper in model/types.rs. All 573
tests pass; clippy is clean.

This is Phase A of improvise-36h. Phase B will mechanically extract
crates/improvise-core/ containing model/, view/, format.rs, workbook.rs.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/draw.rs

@@ -16,7 +16,6 @@ use ratatui::{
     widgets::{Block, Borders, Clear, Paragraph},
 };
 
-use crate::model::Model;
 use crate::ui::app::{App, AppMode};
 use crate::ui::category_panel::CategoryContent;
 use crate::ui::formula_panel::FormulaContent;
@@ -51,13 +50,13 @@ impl<'a> Drop for TuiContext<'a> {
 }
 
 pub fn run_tui(
-    model: Model,
+    workbook: crate::workbook::Workbook,
     file_path: Option<PathBuf>,
     import_value: Option<serde_json::Value>,
 ) -> Result<()> {
     let mut stdout = io::stdout();
     let mut tui_context = TuiContext::enter(&mut stdout)?;
-    let mut app = App::new(model, file_path);
+    let mut app = App::new(workbook, file_path);
 
     if let Some(json) = import_value {
         app.start_import_wizard(json);
@@ -193,7 +192,10 @@ fn draw_title(f: &mut Frame, area: Rect, app: &App) {
         .and_then(|n| n.to_str())
         .map(|n| format!(" ({n})"))
         .unwrap_or_default();
-    let title = format!("  improvise  ·  {}{}{} ", app.model.name, file, dirty);
+    let title = format!(
+        "  improvise  ·  {}{}{} ",
+        app.workbook.model.name, file, dirty
+    );
     let right = " ?:help  :q quit ";
     let line = fill_line(title, right, area.width);
     f.render_widget(
@@ -234,19 +236,20 @@ fn draw_content(f: &mut Frame, area: Rect, app: &App) {
 
         if app.formula_panel_open {
             let a = Rect::new(side.x, y, side.width, ph);
-            let content = FormulaContent::new(&app.model, &app.mode);
+            let content = FormulaContent::new(&app.workbook.model, &app.mode);
             f.render_widget(Panel::new(content, &app.mode, app.formula_cursor), a);
             y += ph;
         }
         if app.category_panel_open {
             let a = Rect::new(side.x, y, side.width, ph);
-            let content = CategoryContent::new(&app.model, &app.expanded_cats);
+            let content =
+                CategoryContent::new(&app.workbook.model, app.workbook.active_view(), &app.expanded_cats);
             f.render_widget(Panel::new(content, &app.mode, app.cat_panel_cursor), a);
             y += ph;
         }
         if app.view_panel_open {
             let a = Rect::new(side.x, y, side.width, ph);
-            let content = ViewContent::new(&app.model);
+            let content = ViewContent::new(&app.workbook);
             f.render_widget(Panel::new(content, &app.mode, app.view_panel_cursor), a);
         }
     } else {
@@ -255,7 +258,9 @@ fn draw_content(f: &mut Frame, area: Rect, app: &App) {
 
     f.render_widget(
         GridWidget::new(
-            &app.model,
+            &app.workbook.model,
+            app.workbook.active_view(),
+            &app.workbook.active_view,
             &app.layout,
             &app.mode,
             &app.search_query,
@@ -267,7 +272,15 @@ fn draw_content(f: &mut Frame, area: Rect, app: &App) {
 }
 
 fn draw_tile_bar(f: &mut Frame, area: Rect, app: &App) {
-    f.render_widget(TileBar::new(&app.model, &app.mode, app.tile_cat_idx), area);
+    f.render_widget(
+        TileBar::new(
+            &app.workbook.model,
+            app.workbook.active_view(),
+            &app.mode,
+            app.tile_cat_idx,
+        ),
+        area,
+    );
 }
 
 fn draw_bottom_bar(f: &mut Frame, area: Rect, app: &App) {
@@ -306,7 +319,7 @@ fn draw_status(f: &mut Frame, area: Rect, app: &App) {
     };
 
     let yank_indicator = if app.yanked.is_some() { " [yank]" } else { "" };
-    let view_badge = format!("  {}{}  ", app.model.active_view, yank_indicator);
+    let view_badge = format!("  {}{}  ", app.workbook.active_view, yank_indicator);
 
     let left = format!("  {}{search_part}  {msg}", mode_name(&app.mode));
     let line = fill_line(left, &view_badge, area.width);