Why: the library now pins eligibility, guards symlinks, reports partial
restores, and bounds the log (Agent-Native#33); the product wires
retention after successful rewinds and needed the HTTP-layer and
eligibility evidence.
Evidence: go test -count=1 ./... green; TestRewindFilesHTTPHappy and
TestRewindFilesHTTPBusy409 (HTTP happy + conflict),
TestBackendCheckpointEligibilityIsWriteFileOnly (the product surface
checkpoints exactly {fs_write_file}).
Impact: checkpoints.jsonl is compacted to the latest 200 anchors after
each successful rewind.
Refs: Agent-Native#33, Agent-Squad#37, N3 plan WP3