Add CEK reactive tests (9/9), fix test runners for CEK-default mode

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>

shared/sx/ref/run_cek_tests.py

@@ -10,9 +10,18 @@ sys.path.insert(0, _PROJECT)
 from shared.sx.parser import parse_all
 from shared.sx.ref import sx_ref
 from shared.sx.ref.sx_ref import (
-    eval_expr, trampoline, make_env, env_get, env_has, env_set,
+    make_env, env_get, env_has, env_set,
     env_extend, env_merge,
 )
+# Use tree-walk evaluator for interpreting .sx test files.
+# The CEK override (eval_expr = cek_run) would cause the interpreted cek.sx
+# to delegate to the transpiled CEK, not the interpreted one being tested.
+# Override both the local names AND the module-level names so that transpiled
+# functions (ho_map, call_lambda, etc.) also use tree-walk internally.
+eval_expr = sx_ref._tree_walk_eval_expr
+trampoline = sx_ref._tree_walk_trampoline
+sx_ref.eval_expr = eval_expr
+sx_ref.trampoline = trampoline
 from shared.sx.types import (
     NIL, Symbol, Keyword, Lambda, Component, Island, Continuation, Macro,
     _ShiftSignal,