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/mod.rs

@@ -0,0 +1,12 @@
+//! Command layer — all model mutations go through this layer so they can be
+//! replayed, scripted, and tested without the TUI.
+//!
+//! Each command is a JSON object: `{"op": "CommandName", ...args}`.
+//! The headless CLI (--cmd / --script) routes through here, and the TUI
+//! App also calls dispatch() for every user action that mutates state.
+
+pub mod dispatch;
+pub mod types;
+
+pub use types::{Command, CommandResult};
+pub use dispatch::dispatch;