Add sub-res8 individual sample drill-down to progressive globe

- New 4th zoom tier: below 120km altitude, switches from H3 clusters
  to individual sample points loaded from lite parquet (60MB vs 280MB)
- Two-stage sample card: instant metadata from lite file, lazy-loaded
  description from full wide parquet on click
- Viewport caching with 30% padding for smooth panning
- Stale-request guards for async camera/query flows
- Hysteresis thresholds (120km enter / 180km exit) to prevent flicker
- Separate PointPrimitiveCollection for samples vs clusters
- Cluster click queries now use lite parquet instead of wide (5x faster)

Data files on R2:
- isamples_202601_samples_map_lite.parquet (60MB, 6M rows, 9 columns)
- Still uses H3 summary files for res4/6/8 cluster view

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: rdhyee