- Store values as int16 (scale 0.1), rounded to DWD's published
  resolution; transpose + Blosc/zstd5 codecs (2.0 MB vs 6.0 MB per chunk)
- Circular mean for wind direction; per-variable units, time convention
  and resolution as coordinates (fixes DWD's Pa/hPa label, documents
  hour-ending rsds)
- Stage months on disk and write one year at a time; track completed
  years in the store; refuse to overwrite without --resume
- --seed-from copies variables from an older archive instead of
  re-downloading them
- Set zarr fill_value to match _FillValue so unwritten hours are NaN
- Require zarr>=3 / Python>=3.11

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>