NUR heading permalinks

NUR replaces Material for MkDocs' default paragraph symbol with a compact link control designed for long-form research reading.

The control:

• appears when a heading is hovered or keyboard-focused;
• remains visible at a reduced opacity on touch devices;
• copies the complete permanent URL to the clipboard;
• displays a temporary confirmation state;
• supports light mode, dark mode and reduced-motion preferences;
• is hidden from printed output;
• uses an embedded SVG mask, so no icon font or binary asset is required.

The implementation lives in:

 docs/assets/stylesheets/nur.css
 docs/assets/javascripts/nur.js

The original heading anchor remains a standard link. If clipboard access is unavailable, NUR falls back to normal hash navigation.