docs: mark Phase 6 (Shuffle Joins) as complete

Author: poyrazK

plans/CPP_MIGRATION_PLAN.md

@@ -6,7 +6,7 @@
 - **Phase 3: Catalog & SQL Parsing** - [x] COMPLETE
 - **Phase 4: Distributed State (Raft)** - [x] COMPLETE
 - **Phase 5: Finalize & Distributed Optimization** - [x] COMPLETE
-- **Phase 6: Multi-Shard Joins (Shuffle Join)** - [/] IN PROGRESS
+- **Phase 6: Multi-Shard Joins (Shuffle Join)** - [x] COMPLETE
 
 ---
 
@@ -49,14 +49,14 @@
     - [x] **Advanced Joins**: Implementation of Broadcast Join POC.
     - [x] **Comprehensive Validation**: 100% test pass on distributed scenarios.
 
-### Phase 6: Multi-Shard Joins (Shuffle Join) [IN PROGRESS]
+### Phase 6: Multi-Shard Joins (Shuffle Join) [COMPLETED]
 - **Goal**: Implement high-throughput data redistribution for distributed joins.
 - **Tasks**:
     - [x] **Context-Aware Buffering**: Isolated staging areas in `ClusterManager`.
     - [x] **Shuffle RPC Handlers**: Implementation of `ShuffleFragment` and `PushData` logic.
     - [x] **Shuffle Orchestration**: Two-phase join coordination in `DistributedExecutor`.
     - [x] **Validation**: Verified orchestration flow via automated integration tests.
-- **Status**: Core orchestration and redistribution logic implemented.
+- **Status**: Production-ready shuffle join infrastructure with robust isolation and error handling.
 
 ---