explorer: surface 'Fetching sample index…' during cold-cache boot→point-mode wait (#190 fix 2)

[rdhyee]

Summary

Implements fix 2 from #190 — surface the loading state during the boot→point-mode gap so cold-cache deep-links to a point-mode altitude don't look broken during the 60-90s wait.

Independent of fix 1 (move off Quarto's bundled DuckDB-WASM 1.24.0), which is blocked on UBIGINT representation changes in newer wasm builds (see spike result comment).

Phase-message sequence

Before (cold-cache deep-link to alt < 120 km):

1. Loading H3 res8... — internal jargon, fires ~immediately, persists ~85 s
2. 175,653 clusters, X samples. Zoom closer for individual samples. — flashes for one frame; misleading because the user is already zoomed in
3. Loading individual samples... — fires from inside loadViewportSamples
4. 47 individual samples. Click one for details.

After:

1. Fetching sample index… — fires ~immediately, persists during the cold-cache wait
2. Loading individual samples...
3. 47 individual samples. Click one for details.

Implementation

• loadRes() now accepts an opts parameter:
  • opts.loadingMsg — override the default "Loading H3 res${res}..." text
  • opts.suppressDoneMsg — skip the interim "... Zoom closer for individual samples." done message
• The camera handler's cluster→point-mode branch (explorer.qmd ~L1848) passes both options, so the boot path shows one coherent message.
• Other loadRes callers (source-filter handler, cluster-mode resolution changes) are unchanged.

Comments inline reference #190 so the rationale is discoverable from the code.

Test plan

• Cold-cache visit to https://isamples.org/explorer.html#v=1&lat=34.9954&lng=33.7052&alt=62054&heading=360.0&mode=point&h3=882da6b2e1fffff (use a fresh browser profile / disabled cache) — confirm Fetching sample index… appears and persists until sample dots render, with no Loading H3 res8... or Zoom closer for individual samples. flashes in between.
• Wider deep-link (alt > 180 km, cluster mode) — confirm phase messages are unchanged (still Loading H3 res${res}... → ${count} clusters, ${samples} samples. Zoom in for finer detail.).
• Source-filter toggle while in cluster mode — confirm Loading H3 res${res}... still appears (loadRes called without opts).
• Source-filter toggle while in point mode — confirm Loading individual samples... still appears.
• Camera zoom from cluster → point mode (warm cache, no cold network wait) — confirm transition still works and Loading individual samples... fires.

Refs

• explorer: 60–90 s 'no dots' window on cold-cache deep-link to point mode (DuckDB-WASM 1.24.0 falls back to full HTTP read) #190 — parent issue (DuckDB-WASM 1.24.0 falls back to full HTTP read; this PR ships the UX-side mitigation independently of the runtime version bump)
• Spike comment — why the version bump can't ship as a one-liner today

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