Follow-up to 2b8ce06c0: that commit's contract update states reclaim skips
the WAL checkpoint entirely when the freelist is empty, but the prior commit
captured the pre-refinement store.go (the re-stage was lost to a .git/index.lock
race). Make the code match the contract: return early on a freelist_count read
error and when freelist == 0, so a steady-state no-op cycle does not run an
extra hourly wal_checkpoint(TRUNCATE).
2f918a3b03