ISO recordings crop or pad a guest's picture when their frame size changes mid-recording #482

Found by the #478 re-review (2026-09-11); pre-existing, and it will now happen more often.

An ISO writer opens LAZILY at a source's first frame, sized to that frame's native dimensions (CLAUDE.md, ISO-1: "sized to that frame's native dims (no scaling)"). If the guest's frame size changes later in the recording, the writer keeps its original size and the picture is cropped or padded.

A guest's frame size can change for several reasons:

• a Zoom resolution change;
• the SDK's downgrade ladder;
• under Multiview: a guest mapped to a slot shows no video until clicked; video subscriptions go to people not on the wall #478's stable tiers, a guest who moves onto or off Program/Preview (720P ↔ 1080P).

That last case makes this more frequent than before.

Fix direction: scale each ISO frame to the writer's opened size on the writer thread (the convert law still holds), or open ISO writers at a fixed 1080p canvas. Verify pixels in the output (CLAUDE.md: verify pixels, not stream presence).