test_week_span_sorting_chronological_by_day sorted its own inline copy
(day-of-month across years), so it kept passing after 7d2a314 made the
real Week sort chronological (full date, then time), and it disagreed
with production. Replace it with a test that calls sort_memories, and
add one for Day, so all three spans are pinned against the real sort.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>