Add GitLab CI pipeline for cross-platform builds
Adds .gitlab-ci.yml with build jobs for Linux x86_64, Linux aarch64, Windows x86_64, macOS (self-hosted), and Android aarch64, plus an automated release job triggered on tag push.
Adds .gitlab-ci.yml with build jobs for Linux x86_64, Linux aarch64, Windows x86_64, macOS (self-hosted), and Android aarch64, plus an automated release job triggered on tag push.