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/grid.rs

@@ -119,29 +119,29 @@ mod tests {
     #[test]
     fn drill_into_formula_cell_returns_data_records() {
         use crate::formula::parse_formula;
-        use crate::model::Model;
         use crate::model::cell::{CellKey, CellValue};
+        use crate::workbook::Workbook;
 
-        let mut m = Model::new("Test");
+        let mut m = Workbook::new("Test");
         m.add_category("Region").unwrap();
-        m.category_mut("Region").unwrap().add_item("East");
-        m.category_mut("_Measure").unwrap().add_item("Revenue");
-        m.category_mut("_Measure").unwrap().add_item("Cost");
-        m.set_cell(
+        m.model.category_mut("Region").unwrap().add_item("East");
+        m.model.category_mut("_Measure").unwrap().add_item("Revenue");
+        m.model.category_mut("_Measure").unwrap().add_item("Cost");
+        m.model.set_cell(
             CellKey::new(vec![
                 ("_Measure".into(), "Revenue".into()),
                 ("Region".into(), "East".into()),
             ]),
             CellValue::Number(1000.0),
         );
-        m.set_cell(
+        m.model.set_cell(
             CellKey::new(vec![
                 ("_Measure".into(), "Cost".into()),
                 ("Region".into(), "East".into()),
             ]),
             CellValue::Number(600.0),
         );
-        m.add_formula(parse_formula("Profit = Revenue - Cost", "_Measure").unwrap());
+        m.model.add_formula(parse_formula("Profit = Revenue - Cost", "_Measure").unwrap());
 
         let layout = make_layout(&m);
         let reg = make_registry();
@@ -376,7 +376,7 @@ impl Cmd for TogglePruneEmpty {
         "toggle-prune-empty"
     }
     fn execute(&self, ctx: &CmdContext) -> Vec<Box<dyn Effect>> {
-        let currently_on = ctx.model.active_view().prune_empty;
+        let currently_on = ctx.view.prune_empty;
         vec![
             Box::new(effect::TogglePruneEmpty),
             effect::set_status(if currently_on {
@@ -454,7 +454,7 @@ impl Cmd for AddRecordRow {
             )];
         }
         // Build a CellKey from the current page filters
-        let view = ctx.model.active_view();
+        let view = ctx.view;
         let page_cats: Vec<String> = view
             .categories_on(crate::view::Axis::Page)
             .into_iter()