fix(csr): write loop locals onto client-rendered for-loop items

A client `data-for` passed its loop locals to hydration in memory but never
wrote the `data-wrn-loop-locals` attribute the SSR path writes. Anything that
resolves locals by READING the DOM -- notably a component's `data-wrn-out-*`
output binding, which calls `decodeLoopLocals(componentRoot)` -- therefore
found nothing and silently dropped the call, with no console error.

A plain DOM handler kept working, because it receives locals through the
hydration closure instead, which is what made the failure look arbitrary: the
same loop variable resolved for `@click` and vanished for a component output.

Both loop paths write the marker now, keyed and non-keyed, so the DOM is the
single source of truth. Encoding goes through UTF-8 before base64 as the
server's does; `btoa` on a raw string throws above U+00FF, which would take the
whole loop down for an ordinary non-ASCII label.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

packages/csr/src/reactive-runtime.ts

@@ -980,6 +980,43 @@ export const REACTIVE_RUNTIME = String.raw`
         renderers[index]();
       }
     }
+    /*
+     * Write loop locals onto a client-rendered item, mirroring the
+     * data-wrn-loop-locals attribute the server emits for an each block.
+     *
+     * Client data-for used to pass locals to hydration in memory only, so
+     * anything that resolves them by READING the DOM -- a component output
+     * binding calls decodeLoopLocals(componentRoot) -- found nothing and
+     * silently dropped the value. A plain DOM handler kept working, because it
+     * receives locals through the hydration closure, which is what made the
+     * failure look arbitrary. The DOM is the single source of truth for
+     * locals; both paths write it now.
+     */
+    function writeLoopLocals(node, locals) {
+      if (!node || node.nodeType !== 1 || !locals) return;
+
+      try {
+        var json = JSON.stringify(locals);
+        if (json === undefined) return;
+
+        var bytes = new TextEncoder().encode(json);
+        var binary = "";
+
+        for (var index = 0; index < bytes.length; index++) {
+          binary += String.fromCharCode(bytes[index]);
+        }
+
+        node.setAttribute("data-wrn-loop-locals", window.btoa(binary));
+      } catch (error) {
+        /*
+         * A value that will not serialise (a cycle, a DOM node) must not take
+         * the whole loop down -- the item still renders, and a handler naming
+         * that local fails on its own terms rather than silently.
+         */
+        console.error("[wrnexus] failed to encode loop locals", error);
+      }
+    }
+
     function decodeLoopLocals(node) {
       if (!node) {
         return {};
@@ -2216,6 +2253,8 @@ export const REACTIVE_RUNTIME = String.raw`
                   itemIndex;
               }
 
+              writeLoopLocals(clone, locals);
+
               hydrateItem(
                 clone,
                 locals,
@@ -2292,6 +2331,7 @@ export const REACTIVE_RUNTIME = String.raw`
               }
 
               var keyedClone = template.cloneNode(true);
+              writeLoopLocals(keyedClone, keyedLocals);
               hydrateItem(keyedClone, keyedLocals);
               record = {
                 node: keyedClone,