Add GitHub Actions CI workflow (replace Travis CI)

[ludomal]

Add GitHub Actions CI, replace Travis CI

Replaces Travis CI integration with GitHub Actions.

This PR:

• Fixes Travis may not catch issues that would arise on recent macs #120 (Travis CI broken)
• Fixes Upgrade TravisCI OSes (and include Windows) #142 (tests not running in CI)

Summary

Travis CI has been non-functional since its free open-source tier was discontinued (2020). The existing .travis.yml references Ubuntu 14.04 (xenial, EOL) and does not run. This PR adds a GitHub Actions workflow that builds and tests on Linux, macOS, and Windows with modern toolchains.

Changes

.github/workflows/ci.yml - New CI workflow

• Linux: GCC + Clang (ubuntu-latest)
• macOS: Clang / Apple Silicon (macos-latest)
• Windows: MSVC (windows-latest), built in Release mode
• LaTeX manual: separate job validating the documentation builds
• Triggers on push/PR to dev and main branches
• 15-minute job timeout
• core.autocrlf=false set before checkout to prevent line-ending issues

Dependencies

This PR should be merged after:

• fix/compilation-errors (build must succeed on modern compilers)
• fix/ctest-portability (test tolerances and Python3 detection)
• fix/uvselp-knr-prototypes (uvselp segfault fix)