Remove deprecated multi-stream infrastructure from backend layer Remove the config type (Only_devices_parallel | For_parallel_copying | Most_parallel_streams), the sharing type (Per_stream | Shared_cross_streams), cross-stream synchronization logic (wait_for_all, shared_writer_streams, host_reading_streams, host_writing_streams, reader_streams, owner_stream), the Streaming_for merge buffer variant, suggested_num_streams from all backends, round_robin/round_robin_dry_run from train.ml, and cross-stream candidate tracking. Replace cross_stream_candidates with constant_buffer_cache for per-device read-only buffer caching. Simplify device_to_device to use phys_equal pointer-identity check instead of the old same-device fast-path guard. Update documentation to reflect the cleanup. Closes #341 Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>