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>

Cargo.toml

@@ -0,0 +1,29 @@
+[package]
+name = "improvise"
+version = "0.1.0"
+edition = "2021"
+description = "Multi-dimensional data modeling terminal application"
+license = "MIT"
+
+[[bin]]
+name = "improvise"
+path = "src/main.rs"
+
+[dependencies]
+ratatui = "0.29"
+crossterm = "0.28"
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"
+anyhow = "1"
+thiserror = "1"
+indexmap = { version = "2", features = ["serde"] }
+chrono = { version = "0.4", features = ["serde"] }
+flate2 = "1"
+unicode-width = "0.2"
+dirs = "5"
+
+[profile.release]
+opt-level = 3
+lto = true
+codegen-units = 1
+strip = true