Optional pre-computed per-tile statistics (count, min, max, sum, mean,
stddev) as plain Parquet columns alongside each band. Enables UDF-free
analytics on any SQL engine — no decompression needed.

- Spec bumped to v0.5.0 with tile statistics section
- CLI: --tile-stats flag for convert raster command
- Validator updated to recognize stats columns
- Docs/changelog updated across site

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