explorer: extend the facet tree to Sampled Feature & Specimen Type (#282 follow-up)

[rdhyee]

The hierarchy tree shipped for Material first (#281/#282). The data pipeline already builds facet_tree_summaries + sample_facet_membership for all three dims (material, context, object_type), and the UI machinery (renderMaterialTreeFacet, materialSelection, syncMaterialTreeVisual, membership filter) generalizes. Extend the tree render + membership filtering to context (Sampled Feature) and object_type (Specimen Type). Source has no vocab tree (stays flat).