Add collection facet to explorer (e.g. OpenContext PKAP) (#243)

[rdhyee]

Resolves #243.

Adds a first-class collection dimension to the explorer: filter to a named
SamplingSite label (e.g. the OpenContext project "PKAP Survey Area") and
layer the existing material / context / object_type facets on top.

Why this design (additive)

"Collection" identity lives on SamplingSite entities, reached only by the
MaterialSampleRecord → produced_by → SamplingEvent → sampling_site → SamplingSite traversal — never on the sample rows the explorer renders. Doing
that array-join live in DuckDB-WASM is the documented in-browser bottleneck, so
membership is precomputed. The current sample_facets_v2 / facet_summaries / facet_cross_filter build pipeline isn't in any repo, so rather than risk
regenerating those, this feature is strictly additive — two new files that
touch nothing existing:

• collections.parquet — dimension (collection_id, label, source, n_samples, centroid_lat/lng, bbox). 61,695 rows, ~3 MB. Powers the top-N
  checkboxes, the search box, and the Featured-Collections preset cameras.
• sample_collections.parquet — membership (pid → collection_id). ~13 MB.
  The filter appends a second pid IN (SELECT … ) subquery in
  facetFilterSQL(), exactly parallel to the existing facet predicate.

A "collection" = a SamplingSite label (≈1,336 site rows share "PKAP Survey
Area"), keyed by a stable hash of (source, label). Verified: PKAP = 15,446
samples.

What's in the PR

• scripts/build_collections.py — builds both files from /current/wide.parquet.
• explorer.qmd — dual-UX collection facet (top-N checkboxes + search-the-tail
  for the ~60K long tail), ?collection= URL param wired through the existing
  facet lifecycle (applyQueryToFacetFilters / writeQueryState /
  handleFacetFilterChange) and the facetFilterSQL() chokepoint.
• collections.qmd — Featured Collections page upgraded to identity-based
  &collection=<id> links + camera fly.
• EXPLORER_STATE.md, data.qmd — document the new param and files.
• tests/test_collections.py — Collections page + explorer facet-DOM checks.

⚠️ Merge gate — requires R2 upload first

The facet is inert until the two files are live on data.isamples.org:

• Run python scripts/build_collections.py --out-dir <dir> --snapshot 202604
• Upload isamples_202604_collections.parquet + isamples_202604_sample_collections.parquet to R2 (behind the data.isamples.org Worker)
• Verify live: open explorer.html?collection=dd74c71982da0e21 → PKAP samples; layer a material facet to confirm it narrows
• Run tests/test_collections.py against the deployed site

Known limitations (v1)

• Collection facet counts are the collection's static total — not
  cross-filtered against other facets (no cross_filter cache for collections).
  The dots and table do respect the filter. Documented in EXPLORER_STATE.md.
• Like the other facets, collection filtering applies at neighborhood/point zoom,
  not to zoomed-out H3 clusters (same #facetNote caveat).

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