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/model/category.rs

@@ -0,0 +1,121 @@
+use indexmap::IndexMap;
+use serde::{Deserialize, Serialize};
+
+pub type CategoryId = usize;
+pub type ItemId = usize;
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct Item {
+    pub id: ItemId,
+    pub name: String,
+    /// Parent group name, if any
+    pub group: Option<String>,
+}
+
+impl Item {
+    pub fn new(id: ItemId, name: impl Into<String>) -> Self {
+        Self { id, name: name.into(), group: None }
+    }
+
+    pub fn with_group(mut self, group: impl Into<String>) -> Self {
+        self.group = Some(group.into());
+        self
+    }
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct Group {
+    pub name: String,
+    /// Parent group name for nested hierarchies
+    pub parent: Option<String>,
+}
+
+impl Group {
+    pub fn new(name: impl Into<String>) -> Self {
+        Self { name: name.into(), parent: None }
+    }
+
+    pub fn with_parent(mut self, parent: impl Into<String>) -> Self {
+        self.parent = Some(parent.into());
+        self
+    }
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct Category {
+    pub id: CategoryId,
+    pub name: String,
+    /// Items in insertion order
+    pub items: IndexMap<String, Item>,
+    /// Named groups (hierarchy nodes)
+    pub groups: Vec<Group>,
+    /// Next item id counter
+    next_item_id: ItemId,
+}
+
+impl Category {
+    pub fn new(id: CategoryId, name: impl Into<String>) -> Self {
+        Self {
+            id,
+            name: name.into(),
+            items: IndexMap::new(),
+            groups: Vec::new(),
+            next_item_id: 0,
+        }
+    }
+
+    pub fn add_item(&mut self, name: impl Into<String>) -> ItemId {
+        let name = name.into();
+        if let Some(item) = self.items.get(&name) {
+            return item.id;
+        }
+        let id = self.next_item_id;
+        self.next_item_id += 1;
+        self.items.insert(name.clone(), Item::new(id, name));
+        id
+    }
+
+    pub fn add_item_in_group(&mut self, name: impl Into<String>, group: impl Into<String>) -> ItemId {
+        let name = name.into();
+        let group = group.into();
+        if let Some(item) = self.items.get(&name) {
+            return item.id;
+        }
+        let id = self.next_item_id;
+        self.next_item_id += 1;
+        self.items.insert(name.clone(), Item::new(id, name).with_group(group));
+        id
+    }
+
+    pub fn add_group(&mut self, group: Group) {
+        if !self.groups.iter().any(|g| g.name == group.name) {
+            self.groups.push(group);
+        }
+    }
+
+    pub fn item_by_name(&self, name: &str) -> Option<&Item> {
+        self.items.get(name)
+    }
+
+    pub fn item_index(&self, name: &str) -> Option<usize> {
+        self.items.get_index_of(name)
+    }
+
+    /// Returns item names in order, grouped hierarchically
+    pub fn ordered_item_names(&self) -> Vec<&str> {
+        self.items.keys().map(|s| s.as_str()).collect()
+    }
+
+    /// Returns unique group names at the top level
+    pub fn top_level_groups(&self) -> Vec<&str> {
+        let mut seen = Vec::new();
+        for item in self.items.values() {
+            if let Some(g) = &item.group {
+                if !seen.contains(&g.as_str()) {
+                    seen.push(g.as_str());
+                }
+            }
+        }
+        seen
+    }
+}