Resumable / delta v3 sync

Make the v3 sync resumable (like GLAM), inside the V3 `SyncConfiguration` + public-R2 connector (#606522 - closed) • Unassigned • 19.4 connector.

• Full-snapshot vs delta selection + per-file checkpoint advance (sync_service.rb); reference malware_pds.rb (logic only, no auth).
• Use Checkpoint sequence/chunk + full_sync_target_sequence (checkpoint.rb).
• Delta recovery + version-range merge: snapshot reapply is idempotent via upsert; delta ingestion needs a retry/recovery design and range merging (PMDB: Exporter: Export SPDX Expressions and Mig... (#596923 - closed) • Orin Naaman • 19.3).

Builds on V3 `SyncConfiguration` + public-R2 connector (#606522 - closed) • Unassigned • 19.4.