docs(adr-150): empirical cross-subject findings — pose-contrastive pretrain refuted

Measured all near-term levers on the official MM-Fi cross-subject split:
- mixup+TTA+ensemble = best at 64.92% (+0.9 over doc 64.04)
- pose-contrastive foundation pretrain: estimated +5..+12, MEASURED -2.3
  (SupCon loss pinned at ln(B) across K/BS/seeds -> same-pose CSI is not
  contrastively alignable across subjects)
- instance-norm+SpecAugment -4.6; CORAL/DANN ~0

Conclusion: the 18-pt in-domain<->cross-subject gap is fundamental subject
shift, not algorithmic. Promotes multi-subject data collection to the primary
lever; recommends re-scoping ADR-150 phase 1 around capture.

Co-Authored-By: claude-flow <ruv@ruv.net>