Adds a new Clustergram body encoding (dotplot) and a dedicated Composition
widget for comparing category proportions/counts across groups as stacked
bars, with a shared control-panel restyle and a round of dendrogram/hover
interaction polish (row dendrogram in composition mode, hover/click
highlight, resize animation, and several stuck-highlight/picking bug fixes).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>