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BackstubeAPI.createDisc() to register discs from Java code (one-line API per loader: fabric Item, forge RegistryObject<Item>, neoforge DeferredItem<Item>)BackstubeMusicDisc.builder() fluent builder with all disc fields plus title(String)/artist(String)/sound(Identifier) shortcutsitem and stack_size fields in disc JSON for custom items and per-disc stack sizesBackstubeAPI.applyDiscData(stack, diskKey, holders) to apply disc marker to any ItemStack