run_tests.ml had a local _scope_stacks hash table, separate from
Sx_primitives._scope_stacks used by the CEK evaluator. SX-level
scope-push!/scope-peek used the local table, but step-sf-context's
scope_peek used the global one. Aser's provide handler pushed to
one table, context read from the other — always got nil.
Fix: alias run_tests.ml's _scope_stacks to Sx_primitives._scope_stacks.
2768/2768 OCaml tests pass. Zero failures.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>