Adds a crash-recovery preamble that restores `final` from `.old` when a
prior swap was interrupted after rename(final, old) but before
rename(tmp, final), and always drops stale `.old`/`.importing` orphans.
Also guards the swap: if nothing new was staged into tmp (empty/missing
staging) and `final` already exists, leave it untouched instead of
swapping in an empty copy. This is required beyond the literal preamble
because the pre-existing swap logic unconditionally replaced `final`
with `tmp` regardless of whether anything was actually imported, which
would otherwise destroy a just-recovered (or any existing) copy whenever
staging is empty.
Jonathan
df7bf2fcf8