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>

shared/static/wasm/sx/core-signals.sx

@@ -149,8 +149,9 @@
       batch
       :effects (mutation)
       (fn
-        ((thunk :as lambda))
+        ((thunk :as callable))
         (set! *batch-depth* (+ *batch-depth* 1))
+        (batch-begin!)
         (cek-call thunk nil)
         (set! *batch-depth* (- *batch-depth* 1))
         (when
@@ -165,14 +166,15 @@
                   ((s :as signal))
                   (for-each
                     (fn
-                      ((sub :as lambda))
+                      ((sub :as callable))
                       (when
                         (not (contains? seen sub))
                         (append! seen sub)
                         (append! pending sub)))
                     (signal-subscribers s)))
                 queue)
-              (for-each (fn ((sub :as lambda)) (sub)) pending))))))
+              (for-each (fn ((sub :as callable)) (sub)) pending))))
+        (batch-end!)))
     (define
       notify-subscribers
       :effects (mutation)

shared/static/wasm/sx_browser.bc.wasm.js

@@ -1792,7 +1792,7 @@
             blake2_js_for_wasm_create: blake2_js_for_wasm_create};
    }
    (globalThis))
-({"link":[["runtime-0db9b496",0],["prelude-d7e4b000",0],["stdlib-23ce0836",[]],["sx-b2cc3269",[2]],["jsoo_runtime-f96b44a8",[2]],["js_of_ocaml-651f6707",[2,4]],["dune__exe__Sx_browser-9b135b3a",[2,3,5]],["std_exit-10fb8830",[2]],["start-f5d3f095",0]],"generated":(b=>{var
+({"link":[["runtime-0db9b496",0],["prelude-d7e4b000",0],["stdlib-23ce0836",[]],["sx-7ca4d3ad",[2]],["jsoo_runtime-f96b44a8",[2]],["js_of_ocaml-651f6707",[2,4]],["dune__exe__Sx_browser-bec4a5c4",[2,3,5]],["std_exit-10fb8830",[2]],["start-f5d3f095",0]],"generated":(b=>{var
 c=b,a=b?.module?.export||b;return{"env":{"caml_ba_kind_of_typed_array":()=>{throw new
 Error("caml_ba_kind_of_typed_array not implemented")},"caml_exn_with_js_backtrace":()=>{throw new
 Error("caml_exn_with_js_backtrace not implemented")},"caml_int64_create_lo_mi_hi":()=>{throw new