clipsync/docs/adr/0003-loop-prevention-key.md

Loop prevention keys on (origin device ID, sequence), not content hash

The original protocol draft used content_hash recent-history dedup as the loop breaker. That is wrong for many-to-many: two different devices pasting the same text are falsely dropped as echoes, and the hash is optional so the gate is bypassable. The loop breaker is now the immutable origin device ID plus a per-peer monotonic sequence: ignore updates whose (origin, sequence) is not newer than the last-seen pair, and drop any update whose origin is the local device. content_hash is retained only for integrity/diagnostics.