refactor(cuda): decompose CustomStorage helper

[hhzhang16]

Summary

Follow-up to #109 and #96 that decomposes the qualified CUDA CustomStorage helper; does not change its protocol or behavior. It does not need to merge in the initial CustomStorage code-freeze window.

Source ownership

flowchart LR
    CLI["main.cpp<br/>CLI dispatch"] --> Server["daemon_server.*<br/>socket, health, shutdown"]
    Server --> CUDA["cuda_operation.*<br/>driver operation + context lifetime"]
    CUDA --> Scheduler["transfer_scheduler.*<br/>workers, deadline, cancellation"]
    Scheduler --> Adapter["transfer_engine.*<br/>Snapshot-local NIXL POSIX adapter"]

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Main changes

• main.cpp: reduced to CLI parsing and command dispatch

• daemon_server: Unix-socket serving, health requests, shutdown, and RPC response handling

• cuda_operation: CUDA initialization, operation completion, and target-scoped primary context lifetime

• transfer_scheduler: per-extent worker creation, shared deadline, sibling cancellation, and
  result aggregation

• Preserves the existing cancellation test and adds scheduler empty-batch, invalid-result, and backend-failure coverage.

• standalone and production Docker targets to build and test

• No change to CUDA operation ordering, timeout behavior, unknown-outcome handling, target identity, or retained-context lifetime.

• Exact-image legacy and POSIX Qwen3-0.6B checkpoint/restore/inference smoke pass

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