When a previous export crashes (OOM, abort, kill), the partially-
downloaded temp export under .dce-temp/ was orphaned. Subsequent
runs started the incremental from the archive's last message ID,
re-downloading everything the failed run had already fetched.
Now scrape_target() checks for orphaned temp exports before each
channel export, salvages truncated JSON (same marker-based repair
as salvage-truncated-export.sh), merges recovered messages into
the archive, and cleans up stale temp dirs. The incremental then
starts from the truly latest message.
Adds salvage-stale smoke test with truncated fixture.
