feat: add view history navigation and drill-into-cell

Add view navigation history with back/forward stacks (bound to < and >).

Introduce CategoryKind enum to distinguish regular categories from
virtual ones (_Index, _Dim) that are synthesized at query time.

Add DrillIntoCell command that creates a drill view showing raw data
for an aggregated cell, expanding categories on Axis::None into Row
and Column axes while filtering by the cell's fixed coordinates.

Virtual categories default to Axis::None and are automatically added
to all views when the model is initialized.

Co-Authored-By: fiddlerwoaroof/git-smart-commit (unsloth/Qwen3.5-35B-A3B-GGUF:Q5_K_M)

src/ui/app.rs

@@ -67,6 +67,11 @@ pub struct App {
     pub yanked: Option<CellValue>,
     /// Tile select cursor (which category index is highlighted)
     pub tile_cat_idx: usize,
+    /// View navigation history: views visited before the current one.
+    /// Pushed on SwitchView, popped by `<` (back).
+    pub view_back_stack: Vec<String>,
+    /// Views that were "back-ed" from, available for forward navigation (`>`).
+    pub view_forward_stack: Vec<String>,
     /// Named text buffers for text-entry modes
     pub buffers: HashMap<String, String>,
     /// Transient keymap for Emacs-style prefix key sequences (g→gg, y→yy, etc.)
@@ -94,6 +99,8 @@ impl App {
             dirty: false,
             yanked: None,
             tile_cat_idx: 0,
+            view_back_stack: Vec::new(),
+            view_forward_stack: Vec::new(),
             buffers: HashMap::new(),
             transient_keymap: None,
             keymap_set: KeymapSet::default_keymaps(),
@@ -125,6 +132,9 @@ impl App {
             cell_key: layout.cell_key(sel_row, sel_col),
             row_count: layout.row_count(),
             col_count: layout.col_count(),
+            none_cats: layout.none_cats.clone(),
+            view_back_stack: self.view_back_stack.clone(),
+            view_forward_stack: self.view_forward_stack.clone(),
             key_code: key,
         }
     }
@@ -184,7 +194,7 @@ impl App {
     /// Hint text for the status bar (context-sensitive)
     pub fn hint_text(&self) -> &'static str {
         match &self.mode {
-            AppMode::Normal => "hjkl:nav  Enter:advance  i:edit  x:clear  t:transpose  /:search  F/C/V:panels  T:tiles  [:]:page  ::cmd",
+            AppMode::Normal => "hjkl:nav  Enter:advance  i:edit  x:clear  t:transpose  /:search  F/C/V:panels  T:tiles  [:]:page  >:drill  ::cmd",
             AppMode::Editing { .. } => "Enter:commit  Esc:cancel",
             AppMode::FormulaPanel => "n:new  d:delete  jk:nav  Esc:back",
             AppMode::FormulaEdit { .. } => "Enter:save  Esc:cancel  — type: Name = expression",

src/ui/effect.rs

@@ -112,10 +112,41 @@ impl Effect for DeleteView {
 pub struct SwitchView(pub String);
 impl Effect for SwitchView {
     fn apply(&self, app: &mut App) {
+        let current = app.model.active_view.clone();
+        if current != self.0 {
+            app.view_back_stack.push(current);
+            app.view_forward_stack.clear();
+        }
         let _ = app.model.switch_view(&self.0);
     }
 }
 
+/// Go back in view history (pop back stack, push current to forward stack).
+#[derive(Debug)]
+pub struct ViewBack;
+impl Effect for ViewBack {
+    fn apply(&self, app: &mut App) {
+        if let Some(prev) = app.view_back_stack.pop() {
+            let current = app.model.active_view.clone();
+            app.view_forward_stack.push(current);
+            let _ = app.model.switch_view(&prev);
+        }
+    }
+}
+
+/// Go forward in view history (pop forward stack, push current to back stack).
+#[derive(Debug)]
+pub struct ViewForward;
+impl Effect for ViewForward {
+    fn apply(&self, app: &mut App) {
+        if let Some(next) = app.view_forward_stack.pop() {
+            let current = app.model.active_view.clone();
+            app.view_back_stack.push(current);
+            let _ = app.model.switch_view(&next);
+        }
+    }
+}
+
 #[derive(Debug)]
 pub struct SetAxis {
     pub category: String,