The first render records an SkPicture before fonts are loaded (fallback
drawText path). When fonts finish loading asynchronously, the cached
picture was never invalidated — subsequent renders replayed stale picture
without paragraph text. Hovering a node forced a full re-render (volatile
overlay path), making text reappear. Un-hovering replayed the stale cache.
Fix: call invalidateScenePicture() at the end of loadFonts() so the next
render records a fresh picture with proper paragraph text.
