docs(study): cross-dataset confirmed on harder NTU-Fi-HumanID task

Re-ran transfer on 14-class person-ID (harder than 6-activity HAR): same
null-transfer result (MM-Fi pretrain 91.7% = random 92.8%). Unified root
cause: CSI in-domain classification lives in the target-trained readout
(random projection already separable); learned reps don't transfer across
subjects/rooms/datasets. WiFi-CSI is distribution-locked. Addresses the
'HAR too easy' caveat.

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