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>

flake.nix

@@ -0,0 +1,62 @@
+{
+  description = "Improvise — Multi-Dimensional Data Modeling Terminal Application";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+    rust-overlay = {
+      url = "github:oxalica/rust-overlay";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs = { self, nixpkgs, rust-overlay, flake-utils }:
+    flake-utils.lib.eachDefaultSystem (system:
+      let
+        overlays = [ (import rust-overlay) ];
+        pkgs = import nixpkgs { inherit system overlays; };
+
+        rustToolchain = pkgs.rust-bin.stable.latest.default.override {
+          extensions = [ "rust-src" "clippy" "rustfmt" ];
+          targets = [ "x86_64-unknown-linux-musl" ];
+        };
+
+        muslCC = pkgs.pkgsMusl.stdenv.cc;
+      in
+      {
+        devShells.default = pkgs.mkShell {
+          nativeBuildInputs = [
+            rustToolchain
+            pkgs.pkg-config
+            # Provide cc (gcc) for building proc-macro / build-script crates
+            # that target the host (x86_64-unknown-linux-gnu).
+            pkgs.gcc
+            # musl-gcc wrapper for the static musl target.
+            muslCC
+          ];
+
+          # Tell Cargo which linker to use for each target so it never
+          # falls back to rust-lld (which can't find glibc on NixOS).
+          CARGO_TARGET_X86_64_UNKNOWN_LINUX_GNU_LINKER =
+            "${pkgs.gcc}/bin/gcc";
+          CARGO_TARGET_X86_64_UNKNOWN_LINUX_MUSL_LINKER =
+            "${muslCC}/bin/cc";
+
+          # Default build target: static musl binary.
+          CARGO_BUILD_TARGET = "x86_64-unknown-linux-musl";
+
+          RUST_BACKTRACE = "1";
+        };
+
+        packages.default =
+          (pkgs.pkgsMusl.makeRustPlatform {
+            cargo = rustToolchain;
+            rustc = rustToolchain;
+          }).buildRustPackage {
+            pname = "improvise";
+            version = "0.1.0";
+            src = ./.;
+            cargoLock.lockFile = ./Cargo.lock;
+          };
+      });
+}