multi_horizon_loader.rs constructs MultiHorizonLoaderConfig with
default_three_scale() (matching alpha_train's production default
and yielding total_positions()=32). The cfg.seq_len=32 override
in loader_stride_4_yields_correct_spacing is now a no-op since
the constructor already provides 32, so it's removed with a comment.
perception_overfit.rs was untouched — it doesn't construct
MultiHorizonLoaderConfig, only PerceptionTrainerConfig (whose own
seq_len field is unrelated to the loader migration).
7abfd3b0b6