Per-layer KL sensitivity from optiq_metadata.json (for by_kl). Mirrors
optiq's read_per_layer_kl: accepts both the top-level per_layer and the
older optimization.per_layer shapes, reads each entry's kl. Returns {}
if the sidecar is absent or records no KL (our own quantizer writes bits
but not kl), in which case by_kl falls back to by_bits.
Per-layer KL sensitivity from
optiq_metadata.json(forby_kl). Mirrors optiq'sread_per_layer_kl: accepts both the top-levelper_layerand the olderoptimization.per_layershapes, reads each entry'skl. Returns{}if the sidecar is absent or records no KL (our own quantizer writesbitsbut notkl), in which caseby_klfalls back toby_bits.