spec: promises — delay/force/delay-force/make-promise/promise?

25 tests pass on OCaml (4357 total) and JS. Promise represented as
mutable dict {:_promise true :forced :thunk :value}; delay-force
adds :_iterative for chain-following semantics.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

spec/evaluator.sx

@@ -2037,8 +2037,10 @@
         nil))))
 
 (define
-  values
-  (fn (&rest vs) (if (= (len vs) 1) (first vs) {:_values true :_list vs})))
+  sf-delay
+  (fn
+    (args env)
+    (let ((thunk (make-lambda (list) (first args) env))) {:forced false :value nil :thunk thunk :_promise true})))
 
 ;; ═══════════════════════════════════════════════════════════════
 ;; Part 11: Entry Points
@@ -2047,6 +2049,42 @@
 ;; eval-expr / trampoline: top-level bindings that override the
 ;; forward declarations from Part 5.
 ;; ═══════════════════════════════════════════════════════════════
+(define
+  sf-delay-force
+  (fn
+    (args env)
+    (let ((thunk (make-lambda (list) (first args) env))) {:_iterative true :forced false :value nil :thunk thunk :_promise true})))
+
+(define promise? (fn (v) (and (dict? v) (get v :_promise false))))
+
+(define make-promise (fn (v) {:forced true :value v :_promise true}))
+
+(define
+  force
+  (fn
+    (p)
+    (if
+      (not (promise? p))
+      p
+      (if
+        (get p :forced false)
+        (get p :value nil)
+        (let
+          ((result (apply (get p :thunk nil) (list))))
+          (let
+            ((final (if (and (get p :_iterative false) (promise? result)) (force result) result)))
+            (dict-set! p :forced true)
+            (dict-set! p :value final)
+            final))))))
+
+(register-special-form! "delay" sf-delay)
+
+(register-special-form! "delay-force" sf-delay-force)
+
+(define
+  values
+  (fn (&rest vs) (if (= (len vs) 1) (first vs) {:_values true :_list vs})))
+
 (register-special-form! "define-type" sf-define-type)
 
 (define