fix(interpreter): contain ${var:?msg} error within subshell boundary

[chaliy]

Summary

• ${var:?msg} errors used ControlFlow::Return which propagated through the subshell boundary, killing the parent shell
• Now both ControlFlow::Exit and ControlFlow::Return are consumed at the subshell boundary, matching real bash behavior

Test plan

• parameter_error_in_subshell_contained — verifies parent shell survives ${var:?msg} in subshell
• Smoke test via CLI confirms end-to-end behavior
• All existing tests pass (199 tests, full suite)

Closes #961