DropSeed = CombatSeed ^ Mix(Galaxy.Seed), derived at construction. A zone with
no galaxy, or a restored seed of 0, keeps its dice bit for bit; CombatSeed
still roots shot, patrol and range-sample dice. RunSeedFixture builds a test
zone's galaxy through RunSave.Commit and the Continue constructor.
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
