Tests: parse "(a (b) (c))" must produce 3 siblings, not nested.
Catches JIT compilation bug where closing parens cause sibling
lists to become children.
Reset sx-parse to CEK on the OCaml kernel — the JIT-compiled
version of sx-parse's complex letrec produces wrong bytecode.
CEK interpretation works correctly (tests pass on all platforms).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>