AUR package distribution

[rhoopr]

Moved from #120.

Create an AUR package (PKGBUILD) for Arch Linux distribution. Either:

• A separate AUR repo (aur/kei)
• Or a PKGBUILD in this repo under dist/

Should build from the release tarball and install the single binary.

Notes

Requires AUR account setup, SSH key registration, and ongoing maintenance (manual or CI-driven updates each release). Not a code-only change.