refactor: migrate callsites to entity types and consolidate config

[yushan8]

Summary

• Orchestrator interface now returns *mapper.GraphChunkReader instead of storage.GraphReader, removing proto leakage from the orchestrator layer
• Controller reads entity.GraphChunk internally, converting to proto via mapper.GraphChunkToProto only at send time
• Controller changed-targets path uses streaming.SplitBySize and streaming.SplitMetadata for chunking
• Consolidates service.streaming.{max_num_targets,...} into a single service.max_message_bytes field (DefaultMaxMessageBytes = 4,250,000)
• Removes old chunking code from core/common (ResultToGetTargetGraphResponse, ChunkMetadata, chunkTargets, splitMap, and associated constants/tests)
• Updates query-bench to use mapper.ResultToGraphChunks

Stack

• PR 1: feat(entity): add compact ID-mapped target graph types #210 (entity types)
• PR 2: feat(mapper): add streaming split, graph chunk mappers, and graphstore adapter #211 (streaming + mappers)
• PR 3: this PR (callsite migration + cleanup)

Test plan

• make build passes
• make test — all 19 test targets pass
• make gazelle — BUILD files in sync
• Controller tests updated to use newGraphChunkReaderFromProto helper
• Orchestrator tests updated for new GraphChunkReader return type
• Orchestrator mock regenerated