Update development plan with Phase 1-3 completion status

- Mark Phase 1 (Protocol Schema Updates) as completed
- Mark Phase 2 (Offset Assignment Logic) as completed
- Mark Phase 3 (Subscription by Offset) as completed
- Add detailed implementation summaries for each completed phase
- Update next steps to focus on Phase 4 (Broker Integration)
- Document comprehensive test coverage (40+ tests) and robust functionality

SMQ_OFFSET_DEVELOPMENT_PLAN.md

@@ -278,10 +278,41 @@ weed/mq/
 
 **Total: 10-16 weeks**
 
+## Implementation Status
+
+- [x] **Phase 1: Protocol Schema Updates** ✅
+  - Updated `mq_schema.proto` with offset fields and offset-based OffsetType enums
+  - Updated `mq_agent.proto` with offset fields in publish/subscribe responses  
+  - Regenerated protobuf Go code
+  - Added comprehensive proto serialization tests
+  - All tests pass, ready for Phase 2
+
+- [x] **Phase 2: Offset Assignment Logic** ✅
+  - Implemented PartitionOffsetManager for sequential offset assignment per partition
+  - Added OffsetStorage interface with in-memory and SQL storage backends
+  - Created PartitionOffsetRegistry for managing multiple partition offset managers
+  - Implemented robust offset recovery from checkpoints and storage scanning
+  - Added comprehensive tests covering assignment, recovery, and concurrency
+  - All tests pass, thread-safe and recoverable offset assignment complete
+
+- [x] **Phase 3: Subscription by Offset** ✅
+  - Implemented OffsetSubscriber for managing offset-based subscriptions
+  - Added OffsetSubscription with seeking, lag tracking, and range operations
+  - Created OffsetSeeker for offset validation and range utilities
+  - Built SMQOffsetIntegration for bridging offset management with SMQ broker
+  - Support for all OffsetType variants and comprehensive error handling
+  - Added extensive test coverage (40+ tests) for all subscription scenarios
+  - All tests pass, providing robust offset-based messaging foundation
+
+- [ ] **Phase 4: Broker Integration**
+- [ ] **Phase 5: SQL Storage Backend**  
+- [ ] **Phase 6: Testing and Validation**
+
 ## Next Steps
 
-1. Review and approve development plan
-2. Set up development branch
-3. Begin Phase 1 implementation
-4. Establish testing and CI pipeline
-5. Regular progress reviews and adjustments
+1. ~~Review and approve development plan~~ ✅
+2. ~~Set up development branch~~ ✅
+3. ~~Begin Phase 1 implementation~~ ✅
+4. Continue with Phase 4: Broker Integration
+5. Establish testing and CI pipeline
+6. Regular progress reviews and adjustments