Port preconnect + preload pattern to Explorer and Deep-Dive (#122)

Follows the "port-out discipline" track: once a pattern works on the
Interactive Explorer (progressive_globe, PR #119), port it to the
other two tutorial pages so they share the win.

- isamples_explorer.qmd: preconnect + preload the 2 KB facet_summaries
  and 6 KB facet_cross_filter parquets (critical-path for instant
  facet rendering).
- zenodo_isamples_analysis.qmd: preconnect + preload the 2 KB
  facet_summaries parquet. The page's primary dataset (wide_h3, 292 MB)
  is intentionally *not* preloaded — DuckDB-WASM uses HTTP range
  requests, and a full preload would waste bandwidth for all visitors.

All three tutorials now benefit from the data.isamples.org Worker's
immutable cache-control (deployed 2026-04-17) — the edge caches served
via these preload fetches are durable across visits.

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

Author: rdhyee

tutorials/isamples_explorer.qmd

@@ -6,6 +6,11 @@ format:
     code-fold: true
     toc: true
     toc-depth: 3
+    include-in-header:
+      text: |
+        <link rel="preconnect" href="https://data.isamples.org" crossorigin>
+        <link rel="preload" as="fetch" crossorigin="anonymous" href="https://data.isamples.org/isamples_202601_facet_summaries.parquet">
+        <link rel="preload" as="fetch" crossorigin="anonymous" href="https://data.isamples.org/isamples_202601_facet_cross_filter.parquet">
 ---
 
 Search and explore **6.7 million physical samples** from scientific collections worldwide.

tutorials/zenodo_isamples_analysis.qmd

@@ -10,6 +10,10 @@ format:
     toc: true
     toc-depth: 3
     theme: cosmo
+    include-in-header:
+      text: |
+        <link rel="preconnect" href="https://data.isamples.org" crossorigin>
+        <link rel="preload" as="fetch" crossorigin="anonymous" href="https://data.isamples.org/isamples_202601_facet_summaries.parquet">
 ---
 
 # Introduction