Function parse_dflash_config 

pub fn parse_dflash_config(json: &str) -> Result<DflashConfig>

Parse a DFlash drafter’s config.json into a DflashConfig. Used by main.rs after fetching the drafter’s HF metadata to size the runtime BlockDiffusionDraftHead (layer count, head_dim, vocab_size, the target_layer_ids capture indices).