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/command/cmd/mod.rs

@@ -21,10 +21,10 @@ pub(super) mod test_helpers {
 
     use crossterm::event::KeyCode;
 
-    use crate::model::Model;
     use crate::ui::app::AppMode;
     use crate::ui::effect::Effect;
     use crate::view::GridLayout;
+    use crate::workbook::Workbook;
 
     use super::core::CmdContext;
     use super::registry::default_registry;
@@ -36,19 +36,21 @@ pub(super) mod test_helpers {
     pub static EMPTY_EXPANDED: std::sync::LazyLock<std::collections::HashSet<String>> =
         std::sync::LazyLock::new(std::collections::HashSet::new);
 
-    pub fn make_layout(model: &Model) -> GridLayout {
-        GridLayout::new(model, model.active_view())
+    pub fn make_layout(workbook: &Workbook) -> GridLayout {
+        GridLayout::new(&workbook.model, workbook.active_view())
     }
 
     pub fn make_ctx<'a>(
-        model: &'a Model,
+        workbook: &'a Workbook,
         layout: &'a GridLayout,
         registry: &'a CmdRegistry,
     ) -> CmdContext<'a> {
-        let view = model.active_view();
+        let view = workbook.active_view();
         let (sr, sc) = view.selected;
         CmdContext {
-            model,
+            model: &workbook.model,
+            workbook,
+            view,
             layout,
             registry,
             mode: &AppMode::Normal,
@@ -72,7 +74,7 @@ pub(super) mod test_helpers {
             display_value: {
                 let key = layout.cell_key(sr, sc);
                 key.as_ref()
-                    .and_then(|k| model.get_cell(k).cloned())
+                    .and_then(|k| workbook.model.get_cell(k).cloned())
                     .map(|v| v.to_string())
                     .unwrap_or_default()
             },
@@ -83,32 +85,32 @@ pub(super) mod test_helpers {
         }
     }
 
-    pub fn two_cat_model() -> Model {
-        let mut m = Model::new("Test");
-        m.add_category("Type").unwrap();
-        m.add_category("Month").unwrap();
-        m.category_mut("Type").unwrap().add_item("Food");
-        m.category_mut("Type").unwrap().add_item("Clothing");
-        m.category_mut("Month").unwrap().add_item("Jan");
-        m.category_mut("Month").unwrap().add_item("Feb");
-        m
+    pub fn two_cat_model() -> Workbook {
+        let mut wb = Workbook::new("Test");
+        wb.add_category("Type").unwrap();
+        wb.add_category("Month").unwrap();
+        wb.model.category_mut("Type").unwrap().add_item("Food");
+        wb.model.category_mut("Type").unwrap().add_item("Clothing");
+        wb.model.category_mut("Month").unwrap().add_item("Jan");
+        wb.model.category_mut("Month").unwrap().add_item("Feb");
+        wb
     }
 
-    pub fn three_cat_model_with_page() -> Model {
-        let mut m = Model::new("Test");
-        m.add_category("Type").unwrap();
-        m.add_category("Month").unwrap();
-        m.add_category("Region").unwrap();
-        m.category_mut("Type").unwrap().add_item("Food");
-        m.category_mut("Type").unwrap().add_item("Clothing");
-        m.category_mut("Month").unwrap().add_item("Jan");
-        m.category_mut("Month").unwrap().add_item("Feb");
-        m.category_mut("Region").unwrap().add_item("North");
-        m.category_mut("Region").unwrap().add_item("South");
-        m.category_mut("Region").unwrap().add_item("East");
-        let view = m.active_view_mut();
-        view.set_axis("Region", crate::view::Axis::Page);
-        m
+    pub fn three_cat_model_with_page() -> Workbook {
+        let mut wb = Workbook::new("Test");
+        wb.add_category("Type").unwrap();
+        wb.add_category("Month").unwrap();
+        wb.add_category("Region").unwrap();
+        wb.model.category_mut("Type").unwrap().add_item("Food");
+        wb.model.category_mut("Type").unwrap().add_item("Clothing");
+        wb.model.category_mut("Month").unwrap().add_item("Jan");
+        wb.model.category_mut("Month").unwrap().add_item("Feb");
+        wb.model.category_mut("Region").unwrap().add_item("North");
+        wb.model.category_mut("Region").unwrap().add_item("South");
+        wb.model.category_mut("Region").unwrap().add_item("East");
+        wb.active_view_mut()
+            .set_axis("Region", crate::view::Axis::Page);
+        wb
     }
 
     pub fn effects_debug(effects: &[Box<dyn Effect>]) -> String {