Fix galaxy, math_rain and strange_attractors (#7), quickstart, real GIFs, 0.2.1

[Mattbusel]

Closes #7.

Left and middle: frames captured on this PC before the change (the blown-out first second, then empty). Right: the same demos after, captured today with PROOF_HIDDEN=1 PROOF_FIXED_DT=60 PROOF_SHOT=....

Why they broke

• life_function is used by the renderer as a glyph scale. galaxy gave every star Orbit { radius: 1..25 } and math_rain gave glyphs Linear { offset: y }, so glyphs were drawn 5 to 25 times their size: the white and cyan walls.
• Strong inverse-square gravity (galaxy), a flow field (math_rain) and seven overlapping attractor fields (strange_attractors) acted on glyphs of mass 0.01 to 0.1, so within a second everything was flung off screen.
• Separately, initial_state(Dadras) sat on an invariant line and decayed to the origin, and initial_state(Rabinovich) diverged to NaN.

Changes

• The three demos now compute positions from their equations every frame: orbits with mild differential rotation (galaxy), falling columns with logistic-map flicker (math_rain), RK4 per point on each attractor fitted to its measured extent (strange_attractors, now on run_ui with labels). Stable indefinitely.
• Standard seeds for Dadras and Rabinovich, plus a unit test that every attractor settles onto a finite non-trivial state.
• ProofEngine::run honours input.quit_requested like run_ui.
• New quickstart example, used verbatim in the README's 3 steps and the crate docs; docs open with a runnable doc test.
• README: plain one-sentence intro, hero GIF, install table, 3 steps, real results; absolute image URLs so the crates.io page renders. GIFs in assets/gifs/, all from hidden-window captures today, each under 3.3 MB. Old 4 MB supernova GIF replaced.
• Site: quickstart code and result frames, galaxy/strange_attractors/math_rain videos.
• Version 0.2.1, CHANGELOG, crate metadata (description, keywords, categories, homepage, documentation).

Tests: sh ci/test-lib.sh 4838 passed locally; doc tests pass.