[TPU Raiden] Backend Tier Extension (Part 1/4): Protocols, Interfaces & Abstract Driver Architecture

[copybara-service]

[TPU Raiden] Backend Tier Extension (Part 1/4): Protocols, Interfaces & Abstract Driver Architecture

Declares the protocol, configuration and driver interfaces that let TPU Raiden
offload and recall KV blocks to a secondary storage tier beyond Host DRAM.
Declarative only: every driver method is a stub returning UnimplementedError,
with the implementations in Part 2.

• Input: BackendConfig carries serving topology as a ParallelismConfig (tp_size,
  tp_rank); tp_rank is required rather than defaulted, so workers in a sharded
  job cannot collide on one shard directory. PosixBackendOptions is the typed
  parse-and-validate point for a POSIX backend.
• Output: PosixPathMapper declares the layout
  <root_dir>/<model_name>/tp<tp_size>_r<tp_rank>///<hash_hex>.bin;
  BlockKeyMapper::MapKey requires a hex-encoded, length-bounded hash.
• Interfaces: the TransferBackendBuffers RPC and BackendTransferSpec in
  worker_service.proto, the pure virtual KVBackend async interface in
  backends/backend.h, and BlockStatus::SHARED_STORAGE.

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