feat(mapper): add entity-proto mappers and streaming splitters

[yushan8]

Summary

• Add internal/streaming/ with SplitBySize (generic wire-size splitter) and SplitMetadata (splits metadata maps across chunks). Proto-free package.
• Add ResultToTargetGraph: converts targethasher.Result to ([]entity.OptimizedTarget, *entity.Metadata)
• Add ChunkTargetGraph: splits targets + metadata into []entity.GetTargetGraphResponse within a byte budget
• Add GetTargetGraphResponseToProto: entity → proto converter for the gRPC boundary
• Add ChangedTargetsResponseToProto: entity → proto converter for the changed-targets gRPC boundary

Test plan

• make build && make test — 19/19 pass
• internal/streaming/streaming_test.go — SplitBySize, SplitMetadata, wire size equivalence
• internal/mapper/target_graph_test.go — ResultToTargetGraph, ChunkTargetGraph, GetTargetGraphResponseToProto round-trip
• make gazelle

Stack

1. feat(entity): add entity types for target graph and changed targets #210 (entity types)
2. feat(mapper): add entity-proto mappers and streaming splitters #211 — this PR (mappers + streaming)
3. refactor: replace proto serialization with JSON and migrate callsites #222 (storage JSON rewrite)
4. refactor: migrate callsites to entity types and consolidate config #212 (callsite migration + cleanup)