Preconnect + preload critical path parquets (#119)

Warms the network for the two parquet files on the phase 1 critical
path so the browser starts fetching them before Cesium is parsed and
DuckDB-WASM asks for them:

- preconnect to data.isamples.org (DNS + TLS early)
- preload the res4 H3 summary (580 KB — primary cluster data)
- preload the facet summaries (2 KB — instant filter counts)

Expected win: 200-500 ms off "nav → phase 1 complete" on cold cache,
less on warm. Measured against the ?perf=1 panel from PR #118.

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

Author: rdhyee

tutorials/progressive_globe.qmd

@@ -3,6 +3,13 @@ title: "Interactive Explorer"
 subtitle: "Search and explore 6.7 million material samples"
 categories: [parquet, spatial, h3, performance, isamples]
 sidebar: false
+format:
+  html:
+    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_h3_summary_res4.parquet">
+        <link rel="preload" as="fetch" crossorigin="anonymous" href="https://data.isamples.org/isamples_202601_facet_summaries.parquet">
 ---
 
 ::: {.callout-note collapse="true"}