Step 10c: batch coalescing + global subscriber registry

Provide subscribers stored in global *provide-subscribers* dict (keyed
by name) instead of on provide frames. Fixes subscriber loss when
frames are reconstructed, and enables cross-cek_run notification.

Batch integration: batch-begin!/batch-end! primitives manage
*provide-batch-depth*. fire-provide-subscribers defers to queue when
depth > 0, batch-end! flushes deduped. signals.sx batch calls both.

context now prefers scope-peek over frame value — scope stack is the
source of truth since provide! always updates it (even in nested
cek_run where provide frames aren't on the kont).

2754/2768 OCaml (14 pre-existing). 32/32 WASM.

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

hosts/ocaml/bin/run_tests.ml

@@ -441,6 +441,8 @@ let make_test_env () =
     match args with
     | [state] -> Sx_ref.cek_run state
     | _ -> Nil);
+  bind "batch-begin!" (fun _args -> Sx_ref.batch_begin_b ());
+  bind "batch-end!" (fun _args -> Sx_ref.batch_end_b ());
   bind "now-ms" (fun _args -> Number 1000.0);
   bind "random-int" (fun args -> match args with [Number lo; _] -> Number lo | _ -> Number 0.0);
   bind "try-rerender-page" (fun _args -> Nil);