fix(core): int to pointer cast in bas proxy

[Genteure]

Fixes #3095

Direct int to pointer cast causing build to fail in some build environments. With docker.io/zmkfirmware/zmk-dev-arm:3.5 it's a warning instead.

Before: https://github.com/Genteure/zmk-config-issue3095/actions/runs/19007070311/job/54282593007#step:11:117
After: https://github.com/Genteure/zmk-config-issue3095/actions/runs/19007544474/job/54283724669#step:11:111

PR check-list

• Branch has a clean commit history
• Additional tests are included, if changing behaviors/core code that is testable.
• Proper Copyright + License headers added to applicable files (Generally, we stick to "The ZMK Contributors" for copyrights to help avoid churn when files get edited)
• Pre-commit used to check formatting of files, commit messages, etc.
• Includes any necessary documentation changes.