Re-add /api/dbg + a frame-gap/long-task probe over the chat-open slide, the innerHTML render time, and
before/after scroll for load-older — to measure on-device where the jank actually is (main-thread block
vs compositor) instead of guessing. Temporary; removed once fixed.
Build marker -> 2026-07-19-batch122.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>