Fix nvexec launch move-only callable forwarding

[fallintoplace]

What changed

• Forward the stored launch callable with the sender's value category.
• Add coverage for a trivially copyable callable that cannot be copied.

Why

nvexec::launch accepts movable callables, but connect() passed the stored callable as an lvalue. Consuming a sender with a move-only callable therefore tried to copy it.

Testing

• git diff --check
• Standalone C++ syntax check for the move-only callable
• CUDA coverage is included for GPU CI

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[ericniebler]

/ok to test 567bc83

[fallintoplace]

@ericniebler Hi, I just pushed the clang format fix. 6d12001

[ericniebler]

/ok to test 6d12001