GlassNavPinnedHost class

Renders the pinned leading and trailing clusters above the Navigator.

A surviving cluster keeps one persistent glass shell whose geometry animates; its element is never remounted mid-morph, because a glass surface's backdrop pass renders fully or not at all and a shell that remounts pops. Clusters that appear or disappear outright materialize instead, over the windows in GlassNavPinnedMetrics.clusterPhaseAt — the effect dissolves the cluster's composited own layer through the shader's own visibility uniform, which is a fade the backdrop pass does honour.