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/category_panel.rs

@@ -0,0 +1,98 @@
+use ratatui::{
+    buffer::Buffer,
+    layout::Rect,
+    style::{Color, Modifier, Style},
+    widgets::{Block, Borders, Widget},
+};
+
+use crate::model::Model;
+use crate::view::Axis;
+use crate::ui::app::AppMode;
+
+pub struct CategoryPanel<'a> {
+    pub model: &'a Model,
+    pub mode: &'a AppMode,
+    pub cursor: usize,
+}
+
+impl<'a> CategoryPanel<'a> {
+    pub fn new(model: &'a Model, mode: &'a AppMode, cursor: usize) -> Self {
+        Self { model, mode, cursor }
+    }
+}
+
+impl<'a> Widget for CategoryPanel<'a> {
+    fn render(self, area: Rect, buf: &mut Buffer) {
+        let is_active = matches!(self.mode, AppMode::CategoryPanel);
+        let border_style = if is_active {
+            Style::default().fg(Color::Cyan)
+        } else {
+            Style::default().fg(Color::DarkGray)
+        };
+
+        let block = Block::default()
+            .borders(Borders::ALL)
+            .border_style(border_style)
+            .title(" Categories [Enter] cycle axis ");
+        let inner = block.inner(area);
+        block.render(area, buf);
+
+        let view = match self.model.active_view() {
+            Some(v) => v,
+            None => return,
+        };
+
+        let cat_names: Vec<&str> = self.model.category_names();
+        if cat_names.is_empty() {
+            buf.set_string(inner.x, inner.y,
+                "(no categories)",
+                Style::default().fg(Color::DarkGray));
+            return;
+        }
+
+        for (i, cat_name) in cat_names.iter().enumerate() {
+            if inner.y + i as u16 >= inner.y + inner.height { break; }
+
+            let axis = view.axis_of(cat_name);
+            let axis_str = match axis {
+                Axis::Row => "Row ↕",
+                Axis::Column => "Col ↔",
+                Axis::Page => "Page ☰",
+                Axis::Unassigned => "none",
+            };
+            let axis_color = match axis {
+                Axis::Row => Color::Green,
+                Axis::Column => Color::Blue,
+                Axis::Page => Color::Magenta,
+                Axis::Unassigned => Color::DarkGray,
+            };
+
+            let cat = self.model.category(cat_name);
+            let item_count = cat.map(|c| c.items.len()).unwrap_or(0);
+
+            let is_selected = i == self.cursor && is_active;
+            let base_style = if is_selected {
+                Style::default().fg(Color::Black).bg(Color::Cyan).add_modifier(Modifier::BOLD)
+            } else {
+                Style::default()
+            };
+
+            // Background fill for selected row
+            if is_selected {
+                let fill = " ".repeat(inner.width as usize);
+                buf.set_string(inner.x, inner.y + i as u16, &fill, base_style);
+            }
+
+            let name_part = format!(" {cat_name} ({item_count})");
+            let axis_part = format!(" [{axis_str}]");
+            let available = inner.width as usize;
+
+            buf.set_string(inner.x, inner.y + i as u16, &name_part, base_style);
+            if name_part.len() + axis_part.len() < available {
+                let axis_x = inner.x + name_part.len() as u16;
+                buf.set_string(axis_x, inner.y + i as u16, &axis_part,
+                    if is_selected { base_style } else { Style::default().fg(axis_color) });
+            }
+        }
+    }
+}