Switch Eigen FetchContent source to GitHub mirror for CI stability

[Copilot]

GitHub Actions builds were failing during CMake configure because Eigen was fetched from GitLab and intermittently unavailable there. This updates the CMake dependency source to a GitHub-hosted mirror while preserving the pinned Eigen version.

• Dependency source update

  • In cmake/eigen.cmake.in, changed Eigen FetchContent_Populate source:
    • from https://gitlab.com/libeigen/eigen
    • to https://github.com/eigen-mirror/eigen
  • Kept GIT_TAG 3.4.0 unchanged to preserve version pinning and dependency behavior.

• Scope

  • Single-file, surgical change to dependency fetch configuration only.
  • No changes to build logic, targets, or project source code.

FetchContent_Populate(
  eigen_proj
  QUIET
  GIT_REPOSITORY  https://github.com/eigen-mirror/eigen
  GIT_TAG         3.4.0
  SOURCE_DIR      eigen
)

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• gitlab.com
  • Triggering command: /usr/lib/git-core/git-remote-https /usr/lib/git-core/git-remote-https origin REDACTED (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

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