explorer: facet counts — remaining work after #157/#226

[rdhyee]

Context

Cross-filter facet counts are already shipped (commit 7cee6a4, 2026-05-01).
The current implementation:

• Cube fast-path for single-axis-single-value selections (reads
  isamples_202601_facet_cross_filter.parquet, 526 cells)
• Live DuckDB-WASM GROUP BY against isamples_202601_sample_facets_v2.parquet
  (5.98M rows) for multi-axis and multi-select-within-axis cases
• 250 ms debounce, facetCountsReqId cancellation token, .recomputing italic
  loading state, .facet-row.zero dimming when count = 0

See explorer.qmd lines 2589-2705 + 977-997 for the implementation.

This issue tracks what's not yet covered.

Open work

1. Bbox-aware counts (design question)

Counts today are global. The legend says "OpenContext: 312,887" regardless of
viewport. In-map H3 summary tiles already show in-viewport counts. Should the
legend mirror that? (Lean: keep legend global — H3 tiles are the viewport-scoped
surface.)

2. Text-search-aware counts

When a search term is active, should facet counts reflect only matching samples?
The facets_url parquet has label, description, place_name columns — an
ILIKE predicate plugs into buildCrossFilterWhere cleanly. Decision needed
on whether to actually do it.

3. Hierarchical concept rollup

context = earthinterior is a sub-concept of anysampledfeature. Does the cube
roll children up to parents, or are counts strictly leaf-level? Need to inspect
the cube generation pipeline and verify against the live query path's behavior.
The two paths could disagree — that's a silent correctness bug.

4. Non-canonical axes

Cube schema covers only source / material / context / object_type. If the
explorer adds project / site / curation-location facets (issue #161 territory),
the cube needs schema migration or those axes fall back to live query only.
Worth deciding the contract before the next axis lands.

5. UX polish (lower priority)

• Threshold for showing "…" instead of just italic on slow queries
• Mobile collapse of long facet lists with counts
• "Reset" affordance when a user has heavily filtered

Out of scope

• Refactoring explorer.qmd (Interactive Explorer rethink: architecture review + UX/feature backlog #163 territory)
• Cube generation pipeline (separate repo)

Acceptance

• Decisions recorded on items 1–4
• Items chosen for implementation broken into sub-issues

Cross-refs: #163, #164, #226. Supersedes #229.