v2.8.6 OSM Download Only

Added

• --download-only — fetch the OSM data to --save-json-file and exit before any
  world generation. Lets an external scheduler (e.g. Meld) pre-fetch a whole region's
  OSM in one request and feed it to many cells via --file, instead of every
  parallel cell querying Overpass and tripping the public per-IP rate limit.

Purely additive — when the flag is omitted, behaviour is identical to v2.8.5.

Usage
arnis --bbox <s,w,n,e> --save-json-file region.json --download-only
arnis --bbox --file region.json --output-dir

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