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/command/keymap.rs

@@ -354,6 +354,10 @@ impl KeymapSet {
         normal.bind(KeyCode::Char('z'), none, "toggle-group-under-cursor");
         normal.bind(KeyCode::Char('H'), none, "hide-selected-row-item");
 
+        // Drill into aggregated cell / view history
+        normal.bind(KeyCode::Char('>'), none, "drill-into-cell");
+        normal.bind(KeyCode::Char('<'), none, "view-back");
+
         // Tile select
         normal.bind(KeyCode::Char('T'), none, "enter-tile-select");
         normal.bind(KeyCode::Left, ctrl, "enter-tile-select");