v0.2.1: the three broken demos work again

cargo add proof-engine or see crates.io/crates/proof-engine.

Fixed

• galaxy, math_rain and strange_attractors no longer draw a blown-out first second and then an empty screen (#7). They now compute positions from their equations every frame and run stable indefinitely.
• initial_state for Dadras (collapsed to the origin) and Rabinovich (diverged) now use standard seeds; a test checks every attractor.
• ProofEngine::run honours input.quit_requested, so Esc works in run demos.

Added

• quickstart example: 5,000 points that chaos tears apart into the Lorenz butterfly, the program from the README's 3 steps.
• Crate docs with a runnable doc test; README and site with GIFs and videos captured from the engine's own framebuffer.

This is a library release with no binaries. The Proof Editor Windows build is still attached to v0.1.0.