Step 10c: bind CEK special form + provide-set frame + scope-stack integration

bind is now a CEK special form that captures its body unevaluated,
establishes a tracking context (*bind-tracking*), and registers
subscribers on provide frames when context reads are tracked.

- bind special form: step-sf-bind, make-bind-frame, bind continue handler
- provide-set frame: provide! evaluates value with kont (fixes peek bug)
- context tracking: step-sf-context appends to *bind-tracking* when active
- scope-stack fallback: provide pushes to scope stack for cek-call contexts
- CekFrame mutation: cf_remaining/cf_results/cf_extra2 now mutable
- Transpiler: subscribers + prev-tracking field mappings, *bind-tracking* in ml-mutable-globals
- Test fixes: string-append → str, restored edge-cases suite

Passing: bind returns initial value, bind with expression, bind with let,
bind no deps is static, bind with conditional deps, provide! updates/multiple/nil,
provide! computed new value, peek read-modify-write, guard inside bind,
bind with string-append, provide! same value does not notify, bind does not
fire on unrelated provide!, bind sees latest value, bind inside provide scope.

Remaining: subscriber re-evaluation on provide! (scope-stack key issue),
batch coalescing (no batch support yet).

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

spec/tests/test-unified-reactive.sx

@@ -106,7 +106,7 @@
       :prefix "item-"
       (assert-equal
         (list "item-a" "item-b")
-        (map (fn (x) (string-append (context :prefix) x)) (list "a" "b")))))
+        (map (fn (x) (str (context :prefix) x)) (list "a" "b")))))
   (deftest
     "context with keyword name"
     (provide :my-key 99 (assert-equal 99 (context :my-key)))))
@@ -260,10 +260,7 @@
       :x 5
       (assert-equal
         "value: 5"
-        (bind
-          (let
-            ((v (context :x)))
-            (string-append "value: " (number->string v)))))))
+        (bind (let ((v (context :x))) (str "value: " v))))))
   (deftest
     "bind no deps is static"
     (let
@@ -532,7 +529,7 @@
     "guard inside bind"
     (provide
       :x 1
-      (assert-equal 1 (bind (guard (exn (#t -1)) (context :x))))))
+      (assert-equal 1 (bind (guard (exn (true -1)) (context :x))))))
   (deftest
     "bind with string-append"
     (provide
@@ -541,4 +538,4 @@
         :second "world"
         (assert-equal
           "hello world"
-          (bind (string-append (context :first) " " (context :second))))))))
+          (bind (str (context :first) " " (context :second))))))))