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/config.toml

@@ -0,0 +1,15 @@
+[build]
+# Always build a fully-static musl binary by default.
+# The devShell sets CARGO_BUILD_TARGET and CARGO_TARGET_*_LINKER,
+# so the linker is resolved automatically via the environment.
+target = "x86_64-unknown-linux-musl"
+
+[target.x86_64-unknown-linux-gnu]
+# Use gcc (not rust-lld) for host-target build scripts on NixOS.
+# rust-lld bundled with rust-overlay can't resolve glibc symbols on NixOS.
+linker = "gcc"
+
+[target.x86_64-unknown-linux-musl]
+# Linker is provided via CARGO_TARGET_X86_64_UNKNOWN_LINUX_MUSL_LINKER
+# in the nix devShell. Outside nix, musl-gcc must be in PATH.
+rustflags = ["-C", "target-feature=+crt-static"]