Step 10c: fix bind subscriber re-evaluation — track names not frames

Root cause: context called inside lambdas (e.g. swap!) went through
nested cek_run with empty kont, so provide frames weren't found and
never tracked to *bind-tracking*.

Three changes in evaluator.sx:
- step-sf-context: track context names (not frames) to *bind-tracking*
  — names work across cek_run boundaries via scope-peek fallback
- bind continue: resolve tracked names to frames via kont-find-provide
  on rest-k before registering subscribers
- subscriber: use empty kont instead of kont-extract-provides — old
  approach created provide frames whose continue handlers called
  scope-pop!, corrupting the scope stack

2752/2768 OCaml tests pass (all 7 bind subscriber tests fixed).
32/32 WASM native tests pass.

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

spec/evaluator.sx

@@ -2745,10 +2745,10 @@
             nil))
         (frame (kont-find-provide kont name)))
       (when
-        (and frame *bind-tracking*)
+        *bind-tracking*
         (when
-          (not (contains? *bind-tracking* frame))
-          (append! *bind-tracking* frame)))
+          (not (contains? *bind-tracking* name))
+          (append! *bind-tracking* name)))
       (make-cek-value
         (if
           frame
@@ -3597,14 +3597,18 @@
                   (prev (get frame "prev-tracking")))
                 (set! *bind-tracking* prev)
                 (let
-                  ((subscriber (fn (fire-kont) (let ((provide-kont (kont-extract-provides fire-kont))) (cek-run (make-cek-state body fenv provide-kont))))))
+                  ((subscriber (fn (fire-kont) (cek-run (make-cek-state body fenv (list))))))
                   (for-each
                     (fn
-                      (pf)
-                      (dict-set!
-                        pf
-                        "subscribers"
-                        (append (get pf "subscribers") (list subscriber))))
+                      (name)
+                      (let
+                        ((pf (kont-find-provide rest-k name)))
+                        (when
+                          pf
+                          (dict-set!
+                            pf
+                            "subscribers"
+                            (append (get pf "subscribers") (list subscriber))))))
                     tracked))
                 (make-cek-value value fenv rest-k)))
             ("provide-set"