chore: register self-hosted CI runners (macOS and Linux/Wayland)

[digital-michael]

Goal

Register self-hosted GitHub Actions runners so CI can build and test on real hardware — particularly for Linux/Wayland (no GitHub-hosted runner supports Wayland natively) and macOS.

Acceptance criteria

• Linux/Wayland runner registered (Centauri — Fedora 43, KWin/Wayland)
• macOS runner registered
• Both runners appear in repo Settings → Actions → Runners as Online
• CI matrix in chore: set up GitHub Actions CI matrix for all platforms #3 uses these runners for platform-specific jobs

Notes

Centauri details: AMD Threadripper 3960X, Fedora 43, kernel 7.0.9, KWin/Wayland.
Runner registration: Settings → Actions → Runners → New self-hosted runner.