Function rope_append_empty_block 

pub fn rope_append_empty_block(store: &Store, block_id: EntityId) -> u32

Append a new empty block to the end of the rope, separating it from any prior content with a \n boundary (only if the rope is already non-empty). Registers block_id at the resulting byte position. Returns that byte position. Used when insert_frame_uc creates a new top-level frame with a single empty block.