fix(cuda-core): require 'static on simt launch_host_function

[Rachit2323]

Fixes #1387

Summary

launch_host_function in cuda-core/src/simt/stream.rs runs the callback later on a driver thread, but it didn't require 'static. So a closure could borrow a local that's already dropped by the time it runs. The version in runtime.rs already has the bound.

Changes

• Added 'static to launch_host_function and callback_wrapper
• Added a compile_fail doc test for a borrowing closure

All existing callers already pass move closures, so nothing else needed changing.

Testing

• just -f cuda-oxide/Justfile check passes (the local mirror of CI: fmt, clippy, tests, guards, docs)
• cargo oxide run <example> passes, or cuda-oxide/scripts/smoketest.sh -o '^<example>$'
• New example added (if applicable)

On macOS (CUDA headers only, no GPU): cargo check, cargo clippy and cargo fmt --check pass for cuda-core and cuda-async. The new doc test passes, and the same snippet compiles on current main, so the test is checking the right thing. Haven't run it on a GPU.

Checklist

• All commits signed off (git commit -s)
• SPDX headers on new source files (no new files)

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