Fix CUDA termination crash via deterministic worker teardown and lazy stream evaluation

[aleroot]

As described in #4506 a termination crash occurs when a detached worker thread outlives the CUDA runtime and attempts to clean up CUDA resources, triggering a std::terminate.

To resolve this I have made some changes so that the worker class is restructured therefore the detached thread only captures a new, inert State struct containing standard threading primitives. Additionally the CUDA resources remain in the Worker, which is now managed by the CommandEncoder as a unique_ptr to ensure deterministic destruction before the runtime unloads. The destructor ~CudaHandle has been modified to safely ignore errors during shutdown, and eval_impl now selects streams lazily to prevent accidental worker instantiation.

Fixes #4506

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