IO registry: defio declares platform suspension points

Core SX has zero IO — platforms extend __io-registry via (defio name
:category :data/:code/:effect ...). The server web platform declares 44
operations in web/io.sx. batchable_helpers now derived from registry
(:batchable true) instead of hardcoded list. Startup validation warns if
bound IO ops lack registry entries. Browser gets empty registry, ready
for step 5 (IO suspension).

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

shared/static/wasm/sx/engine.sx

@@ -117,7 +117,7 @@
   build-request-headers
   :effects (io)
   (fn
-    (el (loaded-components :as list) (css-hash :as string))
+    (el (loaded-components :as list))
     (let
       ((headers (dict "SX-Request" "true" "SX-Current-URL" (browser-location-href))))
       (let
@@ -126,7 +126,6 @@
       (let
         ((comp-hash (dom-get-attr (dom-query "script[data-components][data-hash]") "data-hash")))
         (when comp-hash (dict-set! headers "SX-Components-Hash" comp-hash)))
-      (when css-hash (dict-set! headers "SX-Css" css-hash))
       (let
         ((extra-h (dom-get-attr el "sx-headers")))
         (when
@@ -162,8 +161,6 @@
       (get-header "SX-Location")
       "replace-url"
       (get-header "SX-Replace-Url")
-      "css-hash"
-      (get-header "SX-Css-Hash")
       "trigger-swap"
       (get-header "SX-Trigger-After-Swap")
       "trigger-settle"