Previously the same fixed 7 fields (mode/hidden_dim/n_blocks/emb_dim/
conditioning/lr/batch_size) were always baked into the name, even for a
vanilla run, and router config wasn't represented at all. Now
default_out_dir_name only includes fields that differ from
DEFAULT_CONFIG, adds router/seed/epochs as candidates, and caps at 6
shown fields with a hashed overflow suffix for heavily-swept configs.
lars