Each file's load_or_predecode + label generation is pure CPU work over
disjoint inputs (snapshots, cfg.horizons, cfg.outcome_label_cost). The
sequential for loop was the parallel-friendly bottleneck — converting
to rayon::par_iter gives ~4-8x speedup on typical 4-8 core hosts.
Combined with SPEED-C's ~400x speedup on the inner Welford loop, total
preload throughput is ~1600-3200x faster than the prior single-threaded
O(W) recompute path.
File order is preserved by par_iter's collect contract. Too-few-snapshots
skips emit a warn during load and resolve to None at collection.
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