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/ui/formula_panel.rs

@@ -0,0 +1,76 @@
+use ratatui::{
+    buffer::Buffer,
+    layout::Rect,
+    style::{Color, Modifier, Style},
+    widgets::{Block, Borders, Widget},
+};
+
+use crate::model::Model;
+use crate::ui::app::AppMode;
+
+pub struct FormulaPanel<'a> {
+    pub model: &'a Model,
+    pub mode: &'a AppMode,
+    pub cursor: usize,
+}
+
+impl<'a> FormulaPanel<'a> {
+    pub fn new(model: &'a Model, mode: &'a AppMode, cursor: usize) -> Self {
+        Self { model, mode, cursor }
+    }
+}
+
+impl<'a> Widget for FormulaPanel<'a> {
+    fn render(self, area: Rect, buf: &mut Buffer) {
+        let is_active = matches!(self.mode, AppMode::FormulaPanel | AppMode::FormulaEdit { .. });
+        let border_style = if is_active {
+            Style::default().fg(Color::Yellow)
+        } else {
+            Style::default().fg(Color::DarkGray)
+        };
+
+        let block = Block::default()
+            .borders(Borders::ALL)
+            .border_style(border_style)
+            .title(" Formulas [n]ew [d]elete ");
+        let inner = block.inner(area);
+        block.render(area, buf);
+
+        let formulas = &self.model.formulas;
+
+        if formulas.is_empty() {
+            buf.set_string(inner.x, inner.y,
+                "(no formulas — press 'n' to add)",
+                Style::default().fg(Color::DarkGray));
+            return;
+        }
+
+        for (i, formula) in formulas.iter().enumerate() {
+            if inner.y + i as u16 >= inner.y + inner.height { break; }
+            let is_selected = i == self.cursor && is_active;
+            let style = if is_selected {
+                Style::default().fg(Color::Black).bg(Color::Yellow).add_modifier(Modifier::BOLD)
+            } else {
+                Style::default().fg(Color::Green)
+            };
+            let text = format!("  {} = {:?}", formula.target, formula.raw);
+            let truncated = if text.len() > inner.width as usize {
+                format!("{}…", &text[..inner.width as usize - 1])
+            } else {
+                text
+            };
+            buf.set_string(inner.x, inner.y + i as u16, &truncated, style);
+        }
+
+        // Formula edit mode
+        if let AppMode::FormulaEdit { buffer } = self.mode {
+            let y = inner.y + inner.height.saturating_sub(2);
+            buf.set_string(inner.x, y,
+                "┄ Enter formula (Name = expr): ",
+                Style::default().fg(Color::Yellow));
+            let y = y + 1;
+            let prompt = format!("> {buffer}█");
+            buf.set_string(inner.x, y, &prompt, Style::default().fg(Color::Green));
+        }
+    }
+}