Block entity types

Declare a block entity type and its valid blocks during Hooks::register, then attach its part:

let press = r.block_entity_type("press")
    .blocks(&["mymod:press", "mymod:big_press"])
    .build();
r.block_entity_part(&press, Press).expect("native block entity type");

The builder qualifies the type ID with the registering mod's namespace. It owns the save ID, wire type and valid blocks; block_entity returns only initial Compound data (or none for an empty compound). It is fired inside the write that creates the entity (LevelChunk.setBlockState) and receives the read-only level, a LevelSide, on a server and on a client of one. Unknown blocks, duplicate types, overlapping membership and attempts to own a minecraft: block are refused at load. Refused native block, item or block entity declarations stop startup, as does unreadable known-components input; the game does not fall back to vanilla content. An explicitly declared mod template block uses its declared type; undeclared template blocks retain their vanilla behavior. Adding mod blocks to vanilla types is not supported by this builder. The hooks about the entity (block_entity_tick, block_entity_update_tag, pre_remove_side_effects, menu_button, menu_quick_move) find it by that declared type; the others (block_entity, screen, the block hooks) by block. The factory is a BlockEntityPart method, not a global Hooks method.

r.block_entity_type_key("other:press") looks up another mod's type, and r.block_entity_type_key("minecraft:chest") looks up a vanilla type. Both return the same opaque BlockEntityTypeKey as the declaration builder: equality and hashing use the shared load-owned registration slot, not the type's blocks. Bare names use the minecraft: namespace. The final registry resolves requests after all declarations and refuses unknown IDs, naming the requesting mod and type. key.id(host) reads the canonical name for diagnostics; a key retains its identity until restart. Use a type ID, not a block ID (minecraft:shulker_box, not minecraft:red_shulker_box). BlockEntityRef::kind(host) returns Option<BlockEntityTypeKey>. living.hurt_and_break(host, slot, amount) changes equipped durability on the server; negative amounts repair. It returns the pre-shrink stack only on break, and None when nothing breaks or the entity or slot is unavailable. Creative players' equipment is exempt. The operation does not fire equip effects.

block_entity.replace_data(host, data) replaces its saved payload, clearing omitted fields while retaining identity and position. Like merge_data, it loads transactionally and synchronizes the change; it returns Err(BlockEntityError::Stale) for an invalidated handle, or Ok(false) for data the entity cannot load or an unchanged result. Lookup keys may answer capabilities for vanilla and native types. block_entity_part accepts only native types and returns BlockEntityPartError::VanillaType immediately for a vanilla key. Native Parts are expanded after every mod has registered, so a Part can target a type declared later.