The Month sort keyed on the display day only. The sort is stable, so a
day's photos kept DB order, merged across libraries, and their times
jumped back and forth. Month now sorts like Week: full date, then
timestamp (undated items last).
The sort moves into sort_memories so it can be tested directly; the new
test covers a multi-library, out-of-order day.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>