The band -> bond derivation moves out of GameData into shared/bandBonds.js,
which tools/build-data.mjs runs per band to write bands.json bondIds
(regenerated with --offline; 9 bands are tied, the build checks the ids).
GameData.bandBondIds reads the field, so the server and the client share
one source. DATA.md documents the field; the test checks the helper on
synthetic bands and that the data matches it.
