Fix isamples_explorer: declare facetSummariesError as mutable (#54)

OJS requires `mutable` keyword for cross-cell variable assignment.
The undeclared assignment was killing the facetSummaries cell,
cascading failures to all downstream filters and queries.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: rdhyee

tutorials/isamples_explorer.qmd

@@ -335,17 +335,22 @@ async function runQuery(sql) {
 }
 ```
 
+```{ojs}
+//| code-fold: true
+mutable facetSummariesError = null
+```
+
 ```{ojs}
 //| code-fold: true
 // Tier 1: Load pre-computed facet summaries (2KB, instant)
 facetSummaries = {
-  facetSummariesError = null;
+  mutable facetSummariesError = null;
   try {
     const rows = await runQuery(`SELECT * FROM read_parquet('${facet_summaries_url}')`);
     return rows;
   } catch (e) {
     console.error("Facet summaries load error:", e);
-    facetSummariesError = e;
+    mutable facetSummariesError = e;
     return [];
   }
 }