The MultiHorizonLoader was calling load_or_predecode_mbp10 on every
next_sequence() call, deserializing millions of MBP-10 snapshots per
sequence. With 8000 sequences and 9 files this gave ~50+ hour
training time on what should be IO-trivial work.
Now keep one file cached (LoadedFile { snapshots, labels_full }),
yield ceil(n_max_sequences / n_files) sequences from it before
advancing. Per-horizon labels are computed once per file load and
sliced cheaply per anchor. With 8000/9: ~890 loads → 9 loads.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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