Documentation errors found while verifying the latest features #436

To do

• rules/shape-draw-start-and-path-morph.md: shape: {"type":"path","data":…} fails validation (the syntax is {"path":{"data":…}}), and expressions use t instead of $t.
• rules/whip-transition.md: easing ease_in_out_cubic does not exist.
• rules/chromatic-aberration.md: the icon example {"type":"icon","name":"zap"} fails; it is "icon":"lucide:zap".
• rules/geometry-safety.md (lines ~45 and 57–61) still shows the positional rustmotion validate scenario.json, which is rejected; SKILL.md (~line 210) shows rustmotion render -o output.mp4 --quiet without -f.
• rules/text-component-parity.md says gradient_text stops use "the same format as view.background": they use position, backgrounds use offset.
• SKILL.md gradient_text section mentions neither stops nor the new angle convention.
• Halo zone interpolation claims (see the materials/backgrounds issue).
• Add a docs check to CI that validates every JSON example of SKILL.md and rules/ against the binary, so these regress no more.

Definition of done

• A test that fails on current main is written first. For anything visual it is a render test (scenario → frame → pixel assertions); tests that build AnimatedProperties by hand are not enough — that is exactly how the regressions listed here slipped through.
• Each item is verified end to end: scenario JSON, rustmotion validate, render, image inspected.
• cargo fmt --all --check, cargo clippy --workspace --all-targets -- -D warnings, cargo test --workspace pass.
• Vocabulary changes update crates/rustmotion/skills/SKILL.md, the relevant rules/ file and the exported schema. A field the validator accepts must have an effect, otherwise the validator rejects it or warns.
• No comments in Rust code (repository rule). Commits follow <type>: <verb> <message>. No AI attribution in commits, PRs or issue comments.