docs(skills): fix the broken examples and add the check that keeps them fixed

[LeadcodeDev]

Closes #436. Part of #438.

Seven examples in SKILL.md and rules/ no longer validated against the binary a
generating model is supposed to produce files for.

File	What was wrong
chromatic-aberration.md	{"type":"icon","name":"zap"} — the field is icon, and it carries the set: "lucide:zap"
shape-draw-start-and-path-morph.md	shape: {"type":"path","data":…} — the schema is {"path":{"data":…}}; and its expression used t, which the evaluator rejects by name (unknown identifier \t`) — it is $t`
motion-path.md	repeated the same path spelling
whip-transition.md	easing ease_in_out_cubic, which does not exist
geometry-safety.md, SKILL.md	positional rustmotion validate scenario.json, and a render without -f — both rejected by the CLI
gradient-quality.md	a background template with no preset
dynamic-depth.md	easing on float_3d, which does not take one
text-component-parity.md, SKILL.md	claimed gradient_text's stops share a background's format. They do not: gradient_text names the position position, a background names it offset, and the validator refuses the other. SKILL.md's gradient_text section mentioned neither stops nor the angle convention

Item 7 (halo interpolation claims) needs no edit: #445 makes the code satisfy what
halo-shapes.md and SKILL.md already said.

The eighth item is the one that matters

every_json_example_in_the_skill_documents_still_validates extracts every fenced
json block from both documents, wraps a bare component in a minimal scenario,
and runs the real validator — schema and attribute checks — over it. 108
examples covered.

Skipped: blocks marked **BAD (several files teach by showing the wrong form
first), blocks carrying an ellipsis placeholder, and transition objects, which are
not components. Filtered out: the errors the wrapper itself causes — a missing
asset path, an animation budget measured against an invented scene length. What
remains is a genuine vocabulary error, named with its file and line:

a generating model reads these documents as the source of truth, and 1 example(s)
no longer validate:
  chromatic-aberration.md:7 — (type=icon): invalid component — would be silently
  dropped at render: missing field `icon`

A first version of the test passed while an example was deliberately broken: it
called validate_scenario only, and the error above comes from
check_component_attrs. Both run now.

Reaching the validator from an integration test needed two functions re-exported
from cli. Exporting the whole commands module instead leaked private types
through cmd_render's signature, which clippy refuses.

cargo fmt --all --check clean · cargo clippy --workspace --all-targets --features rustmotion/studio -D warnings clean.