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>

hosts/ocaml/transpiler.sx

@@ -285,7 +285,7 @@
 
 (define
   ml-mutable-globals
-  (list "*strict*" "*prim-param-types*" "*last-error-kont*"))
+  (list "*strict*" "*prim-param-types*" "*last-error-kont*" "*bind-tracking*"))
 
 (define
   ml-is-mutable-global?
@@ -539,6 +539,8 @@
                 (ef "results")
                 (some (fn (k) (= k "raw-args")) items)
                 (ef "raw-args")
+                (some (fn (k) (= k "subscribers")) items)
+                (ef "subscribers")
                 :else "Nil")
               "; cf_extra = "
               (cond
@@ -562,6 +564,8 @@
                 (ef "update-fn")
                 (some (fn (k) (= k "head-name")) items)
                 (ef "head-name")
+                (some (fn (k) (= k "prev-tracking")) items)
+                (ef "prev-tracking")
                 (some (fn (k) (= k "extra")) items)
                 (ef "extra")
                 :else "Nil")