fix(engine,core): wait for async timelines and force rebind before capture

Two fixes for compositions that register timelines after async data
loading (e.g. fetch for TopoJSON map data):

1. engine/frameCapture: remove the hosts.length <= 1 early return
   so the timeline readiness poll runs for ALL compositions, not just
   multi-composition galleries. Single-composition pages with async
   setup were silently skipped.

2. core/runtime/init: expose window.__hfForceTimelineRebind() which
   resets childrenBound and re-runs bindRootTimelineIfAvailable().
   The renderer calls this after all timelines are confirmed present,
   ensuring the root player discovers late-registered timelines from
   fetch callbacks.

Without these fixes, compositions using fetch() to load data at
runtime would render blank frames because the root player bound
timelines before the async setup completed, and seek() never
reached the unbound composition timeline.

packages/core/src/runtime/init.ts

@@ -964,6 +964,11 @@ export function initSandboxRuntimeModular(): void {
     return true;
   };
 
+  (window as any).__hfForceTimelineRebind = () => {
+    childrenBound = false;
+    bindRootTimelineIfAvailable();
+  };
+
   const emitRootStageLayoutDiagnostics = () => {
     const rootNode = resolveRootCompositionElement();
     if (!(rootNode instanceof HTMLElement)) {