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/core.rs

@@ -7,11 +7,18 @@ use crate::model::Model;
 use crate::model::cell::{CellKey, CellValue};
 use crate::ui::app::AppMode;
 use crate::ui::effect::{Effect, Panel};
-use crate::view::{Axis, GridLayout};
+use crate::view::{Axis, GridLayout, View};
+use crate::workbook::Workbook;
 
 /// Read-only context available to commands for decision-making.
+///
+/// Commands receive a `&Model` (pure data) and a `&View` (the active view).
+/// The full `&Workbook` is also available for the rare command that needs
+/// to enumerate all views (e.g. the view panel).
 pub struct CmdContext<'a> {
     pub model: &'a Model,
+    pub workbook: &'a Workbook,
+    pub view: &'a View,
     pub layout: &'a GridLayout,
     pub registry: &'a CmdRegistry,
     pub mode: &'a AppMode,