Fix count universe inconsistency and blank-value mismatch

Codex review found two bugs:

1. facet_summaries counted all 6.68M records but sample_facets only
   had the 5.98M with coordinates — counts jumped when toggling filters.
   Regenerated all three parquet files from the same base universe
   (lat IS NOT NULL). SESAR now consistently 4,389,231 across all files.

2. Baseline summaries included blank-string facet values, but on-the-fly
   queries excluded them with != ''. Regenerated summaries now exclude
   blanks, matching the on-the-fly behavior.

Also: removed dead getDisplayCounts(), fixed stale 0.3MB comment,
added missing quote escaping on source cache lookup.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: rdhyee

tutorials/isamples_explorer.qmd

@@ -34,7 +34,7 @@ facet_summaries_url = "https://data.isamples.org/isamples_202601_facet_summaries
 // Pre-computed cross-filter cache (6KB - instant single-filter lookups)
 cross_filter_url = "https://data.isamples.org/isamples_202601_facet_cross_filter.parquet"
 
-// Slim facets file for on-the-fly multi-filter queries (0.3MB - URI strings, not BIGINT FKs)
+// Facets file for on-the-fly multi-filter queries (63MB - URI strings, not BIGINT FKs)
 sample_facets_url = "https://data.isamples.org/isamples_202601_sample_facets_v2.parquet"
 
 // Source color scheme (consistent with iSamples conventions)
@@ -485,7 +485,7 @@ crossFilteredFacets = {
       const conditions = ["filter_source IS NULL", "filter_material IS NULL",
                           "filter_context IS NULL", "filter_object_type IS NULL"];
       if (activeSources.length === 1)
-        conditions[0] = `filter_source = '${activeSources[0]}'`;
+        conditions[0] = `filter_source = '${activeSources[0].replace(/'/g, "''")}'`;
       else if (activeMaterials.length === 1)
         conditions[1] = `filter_material = '${activeMaterials[0].replace(/'/g, "''")}'`;
       else if (activeContexts.length === 1)
@@ -547,24 +547,6 @@ crossFilteredFacets = {
 }
 ```
 
-```{ojs}
-//| code-fold: true
-// Merge cross-filtered counts with baseline facets
-// Baseline provides the full list of values; cross-filter overrides counts
-function getDisplayCounts(facetKey) {
-  const baseline = facetsByType[facetKey] || [];
-  if (!crossFilteredFacets || !crossFilteredFacets[facetKey]) return baseline;
-
-  const filtered = crossFilteredFacets[facetKey];
-  const countMap = new Map(filtered.map(r => [r.value, r.count]));
-
-  return baseline.map(item => ({
-    ...item,
-    count: countMap.has(item.value) ? countMap.get(item.value) : 0,
-  }));
-}
-```
-
 ```{ojs}
 //| code-fold: true
 // Update facet count labels in-place when cross-filtered counts change