{%- comment -%}
Site-wide animated constellation background.
Include once, as early in as possible. For every page, put it in
_layouts/default.html above {{ content }} rather than in index.html.
Renders to a fixed, pointer-transparent canvas behind all content.
Honours prefers-reduced-motion (static field, no drift) and pauses
entirely when the tab is hidden.
{%- endcomment -%}