Summary

Extract a V3 engine abstraction layer (MutationService, MutationEngine, TableBinding) between the V3 API and V2 engine internals (Graph, Label, Wal, Cdc). This enables swapping V3 engine implementations without modifying core mutation logic.

Closes #200

Before

Controller → V3MutationService → Graph/Label/Wal/Cdc (direct V2 calls)

• V3MutationService was a 298-line God class: Edge/MultiEdge branching, V2 type conversion, WAL/CDC, error handling all mixed together
• EdgeMutationStatus, MultiEdgeMutationStatus, MutationStatus — 3 redundant result types
• MutationEvent<K> generic — Edge and MultiEdge diverge via separate type parameters

After

Controller → MutationService → MutationEngine(interface) → V2BackedEngine → Graph/Label/Wal/Cdc
                                                         → V3Engine (planned)

• MutationService (97 lines): generic mutation orchestration, no Edge/MultiEdge distinction
• MutationEngine + TableBinding: interfaces with no V2 dependencies
• V2BackedEngine, V2BackedTableBinding, V2BackedMessageBinding: V2 implementations
• MutationResult, MutationKey: unified result/key types

Changes

• Add MutationEngine interface: storage/messaging abstraction (WAL, CDC, timeout)
• Add TableBinding interface: per-table mutation capabilities (lock, read, write, error handling)
• Add MutationService: generic mutation orchestrator handling Edge/MultiEdge uniformly
• Introduce MutationKey sealed class: unify SourceTarget (Edge) and Id (MultiEdge) into a single key type
• Introduce MutationResult: consolidate 3 status types into one
• Add UnresolvedEvent interface: replace MutationEvent.Source<E> generic
• Add V2BackedEngine, V2BackedTableBinding, V2BackedMessageBinding: V2 implementations of the above abstractions
• Extract MutationModeContext: isolate mutation mode decision logic
• Move EdgeRecordMapper and lockTimeout to GraphDefaults for constructor injection

Removed

• V3MutationService → MutationService
• V3CompatibleTableBinding → V2BackedTableBinding
• EdgeMutationStatus, MultiEdgeMutationStatus, MutationStatus → MutationResult

Reviewer Notes

• No behavioral changes: only the controller-level API signature changes to MutationResult; actual mutation/messaging behavior is identical
• Suggested review order: MutationEngine → TableBinding → MutationService → V2 implementations (V2BackedEngine → V2BackedTableBinding → V2BackedMessageBinding) → verify deleted files

Checklist

• Small, focused PR (under 500 lines excluding tests); or Draft with a split plan
• Related issue linked or context added in description
• Self-reviewed the diff; commented on tricky parts
• Tests added/updated and passing locally
• Build and lint checks run
• Breaking changes documented with migration notes
• Performance impact reviewed; benchmarks added if needed