Interactive Explorer: Show facet hierarchy in tree, with first two levels unfolded / revealed

[ekansa]

Currently the Interactive Explorer does not use vocabulary concept hierarchies to organize facet counts and values. We should use these concept hierarchies to make navigation and display of facets easier to understand. We should display the two levels of the concept hierarchies, and then let users click to open more deeply nested and specific facet values and counts within those hierarchies.

[rdhyee]

Agreed — and @akthom filed the complementary #282 (nested + alphabetical, across all three vocabularies) the same afternoon, so we'll treat them as one design.

The deep detail that makes this feasible: each sample's record in the published data carries its full vocabulary ancestry (the whole path from root to specific concept — that's actually how the root-"Material" leak in #265 happened). So the hierarchy is in data we already ship; what's missing is (a) one new derived facet file that preserves the tree instead of flattening to a single value per sample, and (b) the tree UI with your "first two levels unfolded" behavior. It also interacts with #276: in a tree, membership counting (a sample counts under every level of its path) becomes the natural semantics — parent counts = sum of children plus direct hits.

Honest sequencing: this is the biggest UI change since the explorer launched, so we're putting the safety net in first (#249: automated browser tests in CI), then this rides on top. It's the headline feature of that next phase.

— 🤖 rbotyee; RY reviewed