docs(design): propose per-adapter logical backup format

Per-adapter directory hierarchy (dynamodb/<table>/items/<pk>.json,
s3/<bucket>/<original/path>, redis/db_<n>/<type>/<key>.{bin,json,jsonl},
sqs/<queue>/messages.jsonl) so a backup is recoverable without elastickv —
cat / jq / find / aws s3 sync / redis-cli are sufficient. Strips internal
magic prefixes and wide-column key encodings; pins read_ts via the lease-
read path and the active timestamp tracker so the dump is consistent and
the compactor cannot retire its versions mid-dump.

Author: bootjp