docs(adr-150): cross-env few-shot — no unsolved deployment case

Decisive capstone: cross-environment (unseen room+people) zero-shot
10.6%, but 5 calibration samples/person -> 60%, 200 -> 73%. The hard
frontier is calibration-soluble, MORE dramatically than cross-subject
(+62.5 vs +12 at K=200). The unsolved-frontier framing was a zero-shot
artifact. Reframes generalization: ship few-shot calibration, not
zero-shot invariance. Recommend accepting ADR-150 re-scoped around the
calibration mechanism.

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