Jade resolves tamed-animal owner names client-side; its Mojang profile
lookup can't resolve offline-mode UUIDs and it falls back to NeoForge's
UsernameCache, which the client loads from <game>/usernamecache.json.
Ship that file (side = "client", installed at the game root) so migrated
owners stop showing as "???".
Maintain the mapping with tools/sync-usernamecache.py after new logins.
The swap does not carry players' explored terrain across. FTB Chunks stores no
rendered map (its region zips hold chunks.dat plus five data PNGs, one of which
packs height into an image channel), and Xaero's region format is closed and now
version 6 with per-pixel NBT blobs, so the community converters no longer work.
Rather than convert pixels, prune a copy of the world save down to exactly the
chunks a player revealed and let Xaero's own generator map it.
Validated against real data: extracting the 167-region / 98,170-chunk mask
cross-checks clean against FTB's own pixel data, and Anvil pruning round-trips
byte-identically (15.7 MB -> 471 KB on a live region file, slot-to-chunk mapping
confirmed against 1024 chunk NBT headers).
Deferred for now: needs the world save and one singleplayer pass per player.
See analysis/FTB-XAERO-MAP-MIGRATION.md.
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