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/command/types.rs

@@ -0,0 +1,99 @@
+use serde::{Deserialize, Serialize};
+
+/// All commands that can mutate a Model.
+///
+/// Serialized as `{"op": "<variant>", ...rest}` where `rest` contains
+/// the variant's fields flattened into the same JSON object.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+#[serde(tag = "op")]
+pub enum Command {
+    /// Add a category (dimension).
+    AddCategory { name: String },
+
+    /// Add an item to a category.
+    AddItem { category: String, item: String },
+
+    /// Add an item inside a named group.
+    AddItemInGroup { category: String, item: String, group: String },
+
+    /// Set a cell value.  `coords` is a list of `[category, item]` pairs.
+    SetCell {
+        coords: Vec<[String; 2]>,
+        #[serde(flatten)]
+        value: CellValueArg,
+    },
+
+    /// Clear a cell.
+    ClearCell { coords: Vec<[String; 2]> },
+
+    /// Add or replace a formula.
+    /// `raw` is the full formula string, e.g. "Profit = Revenue - Cost".
+    /// `target_category` names the category that owns the formula target.
+    AddFormula { raw: String, target_category: String },
+
+    /// Remove a formula by its target name.
+    RemoveFormula { target: String },
+
+    /// Create a new view.
+    CreateView { name: String },
+
+    /// Delete a view.
+    DeleteView { name: String },
+
+    /// Switch the active view.
+    SwitchView { name: String },
+
+    /// Set the axis of a category in the active view.
+    /// axis: "row" | "column" | "page"
+    SetAxis { category: String, axis: String },
+
+    /// Set the page-axis selection for a category.
+    SetPageSelection { category: String, item: String },
+
+    /// Toggle collapse of a group in the active view.
+    ToggleGroup { category: String, group: String },
+
+    /// Save the model to a file path.
+    Save { path: String },
+
+    /// Load a model from a file path (replaces current model).
+    Load { path: String },
+
+    /// Export the active view to CSV.
+    ExportCsv { path: String },
+
+    /// Import a JSON file via the analyzer (non-interactive, uses auto-detected proposals).
+    ImportJson {
+        path: String,
+        model_name: Option<String>,
+        /// Dot-path to the records array (empty = root)
+        array_path: Option<String>,
+    },
+}
+
+/// Inline value for SetCell
+#[derive(Debug, Clone, Serialize, Deserialize)]
+#[serde(untagged)]
+pub enum CellValueArg {
+    Number { number: f64 },
+    Text { text: String },
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct CommandResult {
+    pub ok: bool,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub message: Option<String>,
+}
+
+impl CommandResult {
+    pub fn ok() -> Self {
+        Self { ok: true, message: None }
+    }
+    pub fn ok_msg(msg: impl Into<String>) -> Self {
+        Self { ok: true, message: Some(msg.into()) }
+    }
+    pub fn err(msg: impl Into<String>) -> Self {
+        Self { ok: false, message: Some(msg.into()) }
+    }
+}