docs(skill): correct the shape spellings and the motion-path claim

[LeadcodeDev]

Closes #192.

What

Two lines in SKILL.md — the file an LLM reads before generating — made the generator produce scenarios the schema rejects. A wrong line there does not produce a warning, it produces invalid output repeatedly.

Shape types. ShapeType is externally tagged, but the skill described the parameterised variants as star "(with points, default 5)", which reads as a sibling field:

$ "shape": "star"
Error: invalid component — would be silently dropped at render:
       invalid type: unit variant, expected struct variant

Replaced the prose with the literal JSON for all eight variants.

motion-path. The skill still said the mechanism does not exist; #177 shipped it. Replaced with a pointer to rules/motion-path.md and a note on the near-miss it should now be preventing — motion-path in style is dropped, motion_path in animation is the effect.

Test

skill_documented_shape_spellings_parse asserts the eight documented spellings parse, and that a bare "star" stays an error — the shape the old wording implied. The skill files are embedded by build.rs, so no copy to sync.

cargo test --workspace green, cargo fmt --check and cargo clippy --all-targets -- -D warnings clean.