Add iSamples Zenodo wide format Cesium demo

- New tutorial: parquet_cesium_isamples_wide.qmd
- Points to R2-hosted Zenodo wide parquet (242 MB, 19.5M rows)
- Uses flattened schema with direct latitude/longitude columns
- Matches Eric's OpenContext wide format schema
- Public URL: https://pub-a18234d962364c22a50c787b7ca09fa5.r2.dev/isamples_202512_wide.parquet

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: rdhyee