Adds per-file labels cache sidecar (CachedLabels struct with
labels_full + outcome_prof_long/short_full + outcome_size_long/short_full +
sigma_k_full + pos_fraction + regime_full). Cache key encodes
(horizons, outcome_label_cost, instrument_filter) so any of those
changing invalidates the cache. The load path also rejects caches whose
regime_full length doesn't match the freshly-loaded snapshot count, as a
defense against re-downloaded quarters whose underlying MBP-10 sidecar
was refreshed but whose labels sidecar wasn't.
Stacks with SPEED-C (~400x on Welford) and SPEED-A (~4-8x parallel):
- First run on a file: pay the existing label-generation cost, write
the cache.
- Subsequent runs: load arrays directly from bincode sidecar — ~1s/file
vs ~3 min/file previously.
Inference-only runs skip the cache write to avoid polluting it with
all-default vectors that would mis-serve a later non-inference run.
Cache key suffix format: 'labels_h<H0>_<H1>_<H2>_cost<HEX>_<FILTER>'
to keep filenames portable (no embedded '.' from f32 formatting) and
preserve exact-float identity via raw-bit encoding of the cost.
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