v0.5.0

Improv v0.5.0

A cross-platform multidimensional spreadsheet (Lotus Improv / Quantrix lineage)
in Rust: differential-dataflow incremental recalculation, embedded (SQLite)
Mentat storage, and four front-ends (CLI, TUI, desktop GUI, HTTP server).

Highlights:
- Engine: DD incremental recalc; Number/Boolean/Text/Date/Enum/Error value lanes;
  standalone/broadcast literals; aggregation + scalar functions.
- Formula language: dimension-aware DSL, date literals, CALL()/SQL() source forms,
  controlled-natural-language translation.
- GUI (egui, NeXTSTEP look-and-feel): margin category tiles, formula bar, tool
  palette, saved views, filters, charts, multi-category-per-axis stacking,
  virtualized large-grid rendering.
- What-if scenarios (first-class, deterministic input overlays).
- External-language functions: Python, R, Julia, WASM (wasmi), Pure-lang.
- SQL connectivity: SQLite, PostgreSQL, DuckDB; connection management with
  out-of-band credentials; live-query refresh with a scheduler daemon.
- Server: JSON HTTP API with bearer-token authentication.
- Storage: model persists as datoms on embedded Mentat.

Quality gate green: fmt, clippy -D warnings, workspace tests, cargo-deny, typos,
cargo doc. MSRV rustc 1.97.