Defer seasonal sim to idle + throttle flow field…

Details

The post-merge GA4 verifier timed out on page navigation (networkidle0)
because the heavier weather sim started at DOMContentLoaded and competed
with page load / content fetches on the main thread.
- Defer the simulation start until after window load and an idle
  callback, so it never contends with initial load, first paint, or the
  page's own fetches.
- Update the curl-noise flow field every other frame (it evolves slowly;
  imperceptible) to roughly halve the per-frame noise cost.
Both are also general improvements for real users (no jank during load).

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