docs(adr-150): subject-scaling study — capture diversity, not volume

Measured cross-subject PCK vs N training subjects: 4->8 = +21pts, but
24->32 = +0.45pt. Saturates ~64%, ~19pt below in-domain. Correction to
'more data': subject-count returns vanish past ~16-20; the residual is
device/room/protocol shift. Re-scope phase-1 capture around DIVERSITY
(rooms/devices/protocols) + few-shot target adaptation, not headcount.

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