Constant FEATURE_DIM 

pub const FEATURE_DIM: usize = _; // 21usize

Fixed feature-vector length. Layout (see encode): [bias, task_kind×7, prompt_bucket_norm, has_tools, tool_count_norm, has_images, session_fail_norm, rung×8] = 1 + 7 + 1 + 1 + 1 + 1 + 1 + 8 = 21.