chore(submodules): bump betlang pointer for stdin-form eval fix

betlang: 646b500..351f2df

Picks up the completed eval removal in betlang/conformance/run_conformance.sh
(the previous pointer bump included only the incomplete patch).

Note: wokelang conformance fix was committed on feat/e2e-validation-tests,
not main, so wokelang's submodule pointer here on main is unchanged and
will be bumped when the feature branch merges.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hyperpolymath