fix(cli): reject inherited motion assertion kinds

[user-github-me]

Motion sidecars can currently resolve assertion kinds through Object.prototype. "__proto__" throws validator is not a function, while "constructor" is accepted as valid and fails later during the browser check, skipping the layout/contrast audits.

Require an own validator-table property before dispatch. These names now produce the existing indexed unknown-kind validation error, so check reports motion_spec_invalid and continues its other browser audits.

Validation:

• Six parser/file-reader regressions fail on main and pass with the fix.
• Motion spec, motion audit, and check command suites: 107 passed.
• Built CLI reproduces both failures. With the fix, each invalid sidecar returns one structured motion error while runtime/layout pass and all five contrast samples run and pass.
• Valid sidecar passes strict lint/check, evaluates 21 motion samples, and passes all five contrast samples.
• CLI build/typecheck, repository lint/format, pre-commit gates, comment checks, deletion guard, and test reachability pass.

[jrusso1020]

Thanks for the careful write-up. The guard is correct, but it only triggers when a motion sidecar uses a built-in object property name like constructor or __proto__ as an assertion kind, which no real authoring flow produces, so we'd rather not take it as a standalone change. — Rames