Isomorphic hydration: skip re-render when server HTML present

sx-mount now checks if the target element has children (server-
rendered HTML). If so, skips the client re-render and only runs
hydration (process-elements, hydrate-islands, hydrate-elements).

This preserves server-rendered CSSX styling and avoids the flash
of unstyled content that occurred when the client replaced the
server HTML before re-rendering.

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

shared/static/scripts/sx-browser.js

@@ -14,7 +14,7 @@
   // =========================================================================
 
   var NIL = Object.freeze({ _nil: true, toString: function() { return "nil"; } });
-  var SX_VERSION = "2026-03-23T15:39:32Z";
+  var SX_VERSION = "2026-03-23T16:18:55Z";
 
   function isNil(x) { return x === NIL || x === null || x === undefined; }
   function isSxTruthy(x) { return x !== false && !isNil(x); }
@@ -4627,16 +4627,12 @@ PRIMITIVES["hoist-head-elements-full"] = hoistHeadElementsFull;
   // sx-mount
   var sxMount = function(target, source, extraEnv) { return (function() {
   var el = resolveMountTarget(target);
-  return (isSxTruthy(el) ? (function() {
+  return (isSxTruthy(el) ? ((isSxTruthy(isEmpty(domChildList(el))) ? (function() {
   var node = sxRenderWithEnv(source, extraEnv);
   domSetTextContent(el, "");
   domAppend(el, node);
-  hoistHeadElementsFull(el);
-  processElements(el);
-  sxHydrateElements(el);
-  sxHydrateIslands(el);
-  return runPostRenderHooks();
-})() : NIL);
+  return hoistHeadElementsFull(el);
+})() : NIL), processElements(el), sxHydrateElements(el), sxHydrateIslands(el), runPostRenderHooks()) : NIL);
 })(); };
 PRIMITIVES["sx-mount"] = sxMount;