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/formula/ast.rs

@@ -0,0 +1,58 @@
+use serde::{Deserialize, Serialize};
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub enum AggFunc {
+    Sum,
+    Avg,
+    Min,
+    Max,
+    Count,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct Filter {
+    pub category: String,
+    pub item: String,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub enum Expr {
+    Number(f64),
+    Ref(String),
+    BinOp(String, Box<Expr>, Box<Expr>),
+    UnaryMinus(Box<Expr>),
+    Agg(AggFunc, Box<Expr>, Option<Filter>),
+    If(Box<Expr>, Box<Expr>, Box<Expr>),
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct Formula {
+    /// The raw formula text, e.g. "Profit = Revenue - Cost"
+    pub raw: String,
+    /// The item/dimension name this formula computes, e.g. "Profit"
+    pub target: String,
+    /// The category containing the target item
+    pub target_category: String,
+    /// The expression to evaluate
+    pub expr: Expr,
+    /// Optional WHERE filter
+    pub filter: Option<Filter>,
+}
+
+impl Formula {
+    pub fn new(
+        raw: impl Into<String>,
+        target: impl Into<String>,
+        target_category: impl Into<String>,
+        expr: Expr,
+        filter: Option<Filter>,
+    ) -> Self {
+        Self {
+            raw: raw.into(),
+            target: target.into(),
+            target_category: target_category.into(),
+            expr,
+            filter,
+        }
+    }
+}