Shared Sync with HLC

Description

Implement synchronization for truly shared resources (ContentIdentity, Tags) using the HLC-based log model. These resources can be modified by any device and need conflict resolution.

Architecture: Log-based sync with Hybrid Logical Clocks for ordering.

Data Classification

Shared Resources (this task):

• Tags: Global tag definitions (no device owner)
• Albums: Collections referencing entries from multiple devices
• UserMetadata: When scoped to ContentIdentity (content-universal)

Device-Owned (separate - state-based):

• Locations: Owned by specific device
• Entries: Owned via location's device
• (Handled by state-based sync, not this task)

Implementation Steps

1. Mark tags/albums as shared in Syncable trait
2. Implement commit_shared() in TransactionManager:
   • Generate HLC
   • Write to database
   • Write to sync.db
   • Broadcast to all peers
3. Implement conflict resolution:
   • Tags: Deterministic UUID from name (merge duplicates)
   • Albums: Union merge for entry lists
   • UserMetadata: HLC ordering for LWW
4. Implement ACK mechanism for log pruning
5. Test concurrent tag creation across devices
6. Test album modification conflicts

Technical Details

• Changes written to sync.db (per-device log)
• Broadcast via SharedChange message with HLC
• Peers apply in HLC order
• ACKs enable aggressive pruning (log stays <1000 entries)

Conflict Examples

Tag Name Collision

Device A: Creates tag "Vacation" → HLC(1000,A)
Device B: Creates tag "Vacation" → HLC(1001,B)

Resolution: Deterministic UUID from name
  Both generate: Uuid::v5(NAMESPACE, "Vacation")
  Same UUID → automatically merge

Album Concurrent Edits

Device A: Adds entry-1 to "Summer" → HLC(1000,A)
Device B: Adds entry-2 to "Summer" → HLC(1001,B)

Resolution: Union merge
  Album contains: [entry-1, entry-2]

Acceptance Criteria

• Tags sync between all peers (tag::Model implements Syncable)
• Collections (albums) sync between all peers (collection::Model implements Syncable)
• UserMetadata sync (both entry-scoped and content-scoped)
• HLC ordering works (applied in peer.rs)
• sync.db per-device log implementation with PeerLog
• ACK mechanism prunes old entries (prune_acked method)
• Integration tests validate tag sync and conflicts (10 tests passing)
• ContentIdentity sync with deterministic UUIDs

References

• core/src/infra/sync/NEW_SYNC.md - Shared resource sync
• HLC: LSYNC-009
• Sync service: LSYNC-010