Initial implementation of Improvise TUI

Multi-dimensional data modeling terminal application with:
- Core data model: categories, items, groups, sparse cell store
- Formula system: recursive-descent parser, named formulas, WHERE clauses
- View system: Row/Column/Page axes, tile-based pivot, page slicing
- JSON import wizard (interactive TUI + headless auto-mode)
- Command layer: all mutations via typed Command enum for headless replay
- TUI: Ratatui grid, tile bar, formula/category/view panels, help overlay
- Persistence: .improv (JSON), .improv.gz (gzip), CSV export, autosave
- Static binary via x86_64-unknown-linux-musl + nix flake devShell
- Headless mode: --cmd '{"op":"..."}' and --script file.jsonl

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/view/axis.rs

@@ -0,0 +1,21 @@
+use serde::{Deserialize, Serialize};
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
+pub enum Axis {
+    Row,
+    Column,
+    Page,
+    /// Not yet assigned
+    Unassigned,
+}
+
+impl std::fmt::Display for Axis {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            Axis::Row => write!(f, "Row ↕"),
+            Axis::Column => write!(f, "Col ↔"),
+            Axis::Page => write!(f, "Page ☰"),
+            Axis::Unassigned => write!(f, "─"),
+        }
+    }
+}