Previously fn.length always returned 0 — so the 'length value' test262 tests
failed. Now js-fn-length inspects the lambda's parameter list (via
lambda-params primitive) and counts non-rest params. For functions/components
and callable dicts it still returns 0 (can't introspect arity in those cases).
6 new unit tests, 520/522 total.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>