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/cell.rs

@@ -15,7 +15,7 @@ mod tests {
             ("Type".to_string(), "Food".to_string()),
             ("Month".to_string(), "Jan".to_string()),
         ]);
-        m.set_cell(key, CellValue::Number(42.0));
+        m.model.set_cell(key, CellValue::Number(42.0));
         let layout = make_layout(&m);
         let reg = make_registry();
         let ctx = make_ctx(&m, &layout, &reg);
@@ -33,7 +33,7 @@ mod tests {
             ("Type".to_string(), "Food".to_string()),
             ("Month".to_string(), "Jan".to_string()),
         ]);
-        m.set_cell(key, CellValue::Number(99.0));
+        m.model.set_cell(key, CellValue::Number(99.0));
         let layout = make_layout(&m);
         let reg = make_registry();
         let ctx = make_ctx(&m, &layout, &reg);
@@ -47,7 +47,7 @@ mod tests {
     #[test]
     fn paste_with_yanked_value_produces_set_cell() {
         let mut m = two_cat_model();
-        m.set_cell(
+        m.model.set_cell(
             CellKey::new(vec![
                 ("Type".into(), "Food".into()),
                 ("Month".into(), "Jan".into()),