The previous commit had this backwards: the sidecar date was only
consulted when the existing date came from an mtime, so a filename date
silently outranked what Instagram itself reported.
The order is sidecar, then filename, then mtime -- metadata first,
mtime last, since mtime is when the file hit disk and says nothing about
when the post was made. Ties keep the incumbent so two equally
authoritative files cannot flip a post's date by scan order.
Extracted to src/lib/post-dates.ts rather than left inline, because the
rule is easy to state and easy to get wrong -- the tests include an
order-independence case that would have caught the original mistake.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
