Fix eval-expr stub: define as CEK wrapper, not error stub

The eval-expr forward declaration was an error-throwing stub that
the CEK fixup was supposed to override. If anything prevented the
fixup from running (or if eval-expr was captured by value before
the fixup), the stub would throw "CEK fixup not loaded".

Fix: define eval-expr and trampoline as real CEK wrappers at the
end of evaluator.sx (after cek-run is defined). The forward
declaration is now a harmless nil-returning stub. The fixup still
overrides with the iterative version, but even without it, eval
works correctly.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

spec/evaluator.sx

@@ -453,11 +453,10 @@
 ;;   trampoline = (val) → if thunk? then eval-expr(thunk-expr, thunk-env) else val
 ;; All evaluation goes through the CEK machine.
 
+;; eval-expr: forward declaration — redefined at end of file after cek-run exists.
+;; This stub is needed so functions between here and Part 3 can reference eval-expr.
 (define eval-expr
-  (fn (expr (env :as dict))
-    ;; Stub — overridden by CEK fixup before any code runs.
-    ;; If this executes, CEK fixup failed to load.
-    (error "eval-expr: CEK fixup not loaded")))
+  (fn (expr (env :as dict)) nil))
 
 
 
@@ -2470,3 +2469,22 @@
       (when sx-text
         (thaw-from-sx sx-text)
         true))))
+
+
+;; **************************************************************************
+;; eval-expr / trampoline — canonical definitions (after cek-run is defined)
+;; **************************************************************************
+;;
+;; These override the forward declarations from Part 2. All evaluation
+;; goes through the CEK machine. The CEK fixups in the host platform
+;; may further override these (e.g., to make cek-run iterative).
+
+(define eval-expr
+  (fn (expr (env :as dict))
+    (cek-run (make-cek-state expr env (list)))))
+
+(define trampoline
+  (fn (val)
+    (if (thunk? val)
+      (eval-expr (thunk-expr val) (thunk-env val))
+      val)))