Many test262 tests write 'assert(condition, msg)' as a plain call, not
'assert.sameValue(...)'. The stub now sets assert.__callable__ to
__assert_call__ so the dispatch in js-call-plain finds a callable.
Also widens verifyNotEnumerable etc. to 5-arg signatures — some tests call
them with (o, name, value, writable, configurable).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>