Wire 15 previously-stubbed coordinator endpoints through gRPC to the
worker service, replacing 501 Not Implemented with real proxy logic.
Only exec_tty (WebSocket) remains as a stub.

Proto layer (worker.proto):
- Add 15 new RPCs: snapshot CRUD (5), clone, export/import (streaming),
  file upload/download (streaming), image pull/list/get, get_execution
- Add ~20 new message types including streaming chunks

Worker service (service.rs):
- Implement all 15 RPC handlers calling BoxliteRuntime
- Add snapshot_info_to_proto and image_info_to_proto converters
- Enhance ActiveExecution with ExecStatus tracking (Running/Completed/Failed)
- Add streaming support for export (server-stream) and import (client-stream)
- Add tar-based file transfer via copy_into/copy_out

Coordinator proxy (proxy.rs):
- Replace 15 stubs with real REST→gRPC proxy implementations
- Add any_active_worker_client helper for non-box-scoped endpoints
- Clone handler registers BoxMapping for cloned box on same worker
- Import handler uses scheduler for worker selection
- Export/download stream gRPC responses as binary HTTP bodies
- Upload/import convert HTTP body to gRPC client-streaming chunks

Tests:
- Update 15 stub tests to verify correct error behavior (404/503)
- Add unit tests for proto conversion helpers and ExecStatus
- 115 tests pass (66 unit + 43 integration + 6 conformance)