measure-lab: clarify live mapping diagnostic and verify frozen rendering #101

SESSION-FORMAT.md overstates displayMappingCheck as proof that frozen taps land on the wrong pixel. CaptureController compares live ARKit mapping with manual portrait mapping; frozen images use a separate clockwise, centered aspect-fill renderer whose inverse is PortraitFillMapping. A disagreement with the live renderer does not prove the frozen renderer is wrong, and a passing comparison at one point does not validate an entire mapping.

Clarify the diagnostic and nearby comment, explicitly state it does not change accepted, and verify rendered frozen taps on a phone before assigning an acceptance policy. No wrong-ray reproduction was established by source review.

Follow-up from PR #7 discussion_r4114098004, dispositioned under the approved convergence rule as a documentation overstatement rather than a verified P1 false-acceptance defect.