fix(producer): preserve inner root element when inlining sub-compositions

The producer's inlineSubCompositions was not passing flattenInnerRoot,
causing sub-composition inner root elements to be unwrapped during
compilation. Scripts using #id selectors (rewritten to
[data-hf-authored-id] by the scoping proxy) could not find their DOM
root because the authored-id attribute was never set.

The core bundler (bundleToSingleHtml) already passed flattenInnerRoot
correctly. This aligns the producer's render compilation with the
same behavior: clone the inner root, strip timing/composition attrs,
replace id with data-hf-authored-id, and mark with
data-hf-inner-root.

packages/producer/src/services/htmlCompiler.ts

@@ -559,6 +559,19 @@ function inlineSubCompositions(
 
   if (!hosts.length) return html;
 
+  const FLATTENED_INNER_ROOT_STRIP_ATTRS = [
+    "data-composition-id",
+    "data-composition-file",
+    "data-start",
+    "data-duration",
+    "data-end",
+    "data-track-index",
+    "data-track",
+    "data-composition-src",
+    "data-hf-authored-duration",
+    "data-hf-authored-end",
+  ];
+
   const result = inlineSubCompositionsShared(
     document as unknown as Document,
     hosts as unknown as Element[],
@@ -574,6 +587,19 @@ function inlineSubCompositions(
         return compHtml;
       },
       parseHtml: (htmlStr: string) => parseHTML(htmlStr).document as unknown as Document,
+      flattenInnerRoot: (innerRoot: Element): Element => {
+        const prepared = innerRoot.cloneNode(true) as Element;
+        const authoredRootId = prepared.getAttribute("id")?.trim();
+        for (const attrName of FLATTENED_INNER_ROOT_STRIP_ATTRS) {
+          prepared.removeAttribute(attrName);
+        }
+        if (authoredRootId) {
+          prepared.removeAttribute("id");
+          prepared.setAttribute("data-hf-authored-id", authoredRootId);
+        }
+        prepared.setAttribute("data-hf-inner-root", "true");
+        return prepared;
+      },
       scriptErrorLabel: "[Compiler] Composition script failed",
     },
   );