test-cek-reactive.sx: 9 tests across 4 suites — deref pass-through,
signal without reactive-reset, reactive-reset shift with continuation
capture, scope disposal cleanup. run_cek_reactive_tests.py: new runner
loading signals+frames+cek. Both test runners override sx_ref.eval_expr
back to tree-walk so interpreted .sx uses tree-walk internally.
Plan page added to sx-docs.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
giles
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