Guard batch-begin!/batch-end! with (client?) — server-only platform ops

These are OCaml-side bookkeeping for the Python async bridge. The browser
WASM kernel registers them in the CEK env but not the VM global table,
so bytecode-compiled batch() crashed with "VM undefined: batch-begin!".
The SX-level *batch-depth*/*batch-queue* already handle batching correctly.

Verified in Playwright sandbox: signal, deref, reset!, batch, computed
all work with source fallback (sxbc load-format issue is pre-existing).

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

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

@@ -151,7 +151,7 @@
       (fn
         ((thunk :as callable))
         (set! *batch-depth* (+ *batch-depth* 1))
-        (batch-begin!)
+        (when (not (client?)) (batch-begin!))
         (cek-call thunk nil)
         (set! *batch-depth* (- *batch-depth* 1))
         (when
@@ -174,7 +174,7 @@
                     (signal-subscribers s)))
                 queue)
               (for-each (fn ((sub :as callable)) (sub)) pending))))
-        (batch-end!)))
+        (when (not (client?)) (batch-end!))))
     (define
       notify-subscribers
       :effects (mutation)

spec/signals.sx

@@ -151,7 +151,7 @@
       (fn
         ((thunk :as callable))
         (set! *batch-depth* (+ *batch-depth* 1))
-        (batch-begin!)
+        (when (not (client?)) (batch-begin!))
         (cek-call thunk nil)
         (set! *batch-depth* (- *batch-depth* 1))
         (when
@@ -174,7 +174,7 @@
                     (signal-subscribers s)))
                 queue)
               (for-each (fn ((sub :as callable)) (sub)) pending))))
-        (batch-end!)))
+        (when (not (client?)) (batch-end!))))
     (define
       notify-subscribers
       :effects (mutation)