thumb_request_bench measures UI-thread time and allocations for grid
thumbnail requests (with and without pending work), using the same counting
allocator as the other bench examples. REAL_PHOTOS=1 samples up to four
photos from PHOTO_ROOT, which must now be set explicitly; the original
defaulted to a local photo library path.
Split from a327b98 (T0 thumbnail work).
Co-authored-by: Codex <noreply@openai.com>
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
