Step 10c: unified reactive model — peek + provide! special forms + tracking primitives

CEK evaluator integration:
- peek — non-tracking read from provide frame (like context but never subscribes)
- provide! — mutate value in provide frame (cf_extra made mutable)
- Both dispatch as special forms alongside provide/context

Scope-stack primitives (for adapter/island use):
- provide-reactive! / provide-pop-reactive! / provide-set! — signal-backed scope
- peek (primitive) — non-tracking scope read
- context (override) — tracking-aware scope read
- bind — tracked computation with auto-resubscription
- tracking-start! / tracking-stop! / tracking-active? — tracking context

12/13 user-authored peek/provide! tests pass.
bind integration with CEK context pending (scope vs kont gap).

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

spec/evaluator.sx

@@ -1506,6 +1506,8 @@
               ("deref" (step-sf-deref args env kont))
               ("scope" (step-sf-scope args env kont))
               ("provide" (step-sf-provide args env kont))
+              ("peek" (step-sf-peek args env kont))
+              ("provide!" (step-sf-provide! args env kont))
               ("context" (step-sf-context args env kont))
               ("emit!" (step-sf-emit args env kont))
               ("emitted" (step-sf-emitted args env kont))
@@ -2708,6 +2710,49 @@
         env
         kont))))
 
+(define
+  step-sf-peek
+  (fn
+    (args env kont)
+    (let
+      ((name (trampoline (eval-expr (first args) env)))
+        (default-val
+          (if
+            (>= (len args) 2)
+            (trampoline (eval-expr (nth args 1) env))
+            nil))
+        (frame (kont-find-provide kont name)))
+      (make-cek-value
+        (if
+          frame
+          (get frame "value")
+          (if
+            (env-has? env "peek")
+            (apply (env-get env "peek") (list name default-val))
+            default-val))
+        env
+        kont))))
+
+(define
+  step-sf-provide!
+  (fn
+    (args env kont)
+    (let
+      ((name (trampoline (eval-expr (first args) env)))
+        (new-val (trampoline (eval-expr (nth args 1) env)))
+        (frame (kont-find-provide kont name)))
+      (if
+        frame
+        (do
+          (dict-set! frame "value" new-val)
+          (make-cek-value new-val env kont))
+        (if
+          (env-has? env "provide-set!")
+          (do
+            (apply (env-get env "provide-set!") (list name new-val))
+            (make-cek-value new-val env kont))
+          (make-cek-value nil env kont))))))
+
 (define
   step-sf-emit
   (fn