Function config_from_gguf_dir 

pub fn config_from_gguf_dir(model_dir: &Path) -> Result<ModelConfig>

2026-09-26: Build a ModelConfig from the .gguf that find_gguf picks in model_dir.

Passes config_from_gguf the trailing ggml dim of token_embd.weight (the vocab) and whether output.weight exists; config_from_gguf treats a missing output.weight as tied embeddings. Errors when there is no .gguf, when the file cannot be opened, mapped or parsed, or when the config cannot be built.