Function mountSidebar

Mounts the sidebar into container and returns a SidebarHandle for external updates and cleanup.

Features:

• Section tree – recursive, collapsible tree mirroring the parsed structure. The first level is expanded by default; deeper levels start collapsed.
• Entropy heatmap – optional per-row bar showing Shannon entropy; toggled via the header button.
• Selection sync – highlights the deepest section that contains the start of the active selection and scrolls it into view.