Added //go:build go1.23 constraint to the unique demo.
This allows Go's build system to automatically skip this demo
on Go versions older than 1.23, avoiding compilation errors
where the unique package doesn't exist.
This approach is cleaner than version checking in test scripts
and leverages Go's native build tag system.
Generated with [codeagent](https://github.com/qbox/codeagent)
Co-authored-by: cpunion <cpunion@users.noreply.github.com>
Added _demo/go/unique with code demonstrating unique.Make usage.
This demo showcases the fix for the compilation panic issue.
Note: The unique package requires full runtime support (weak pointers, etc.)
which is still under development. The test_demo.sh script should be
updated separately to skip this demo on Go 1.23 or earlier.
Generated with [codeagent](https://github.com/qbox/codeagent)
Co-authored-by: cpunion <cpunion@users.noreply.github.com>
