[zfergus]

Description

Lets the Eigen CPM recipe (cmake/recipes/eigen.cmake) fall back to the GitHub mirror when GitLab can't be reached. Cloning from GitLab fails often enough in CI to break the wheel builds.

• Before CPMAddPackage runs, a git ls-remote call (30 s timeout) checks that GitLab has the tag. If it doesn't answer, the recipe warns and clones from eigen-mirror/eigen instead.
  • A failed clone inside CPMAddPackage is a fatal error and can't be retried, so the source has to be picked before CPM runs.
• The check is skipped when nothing needs downloading: Eigen is already in the CPM cache or _deps, CPM_eigen_SOURCE is set, or FETCHCONTENT_FULLY_DISCONNECTED is on.
• CUSTOM_CACHE_KEY 5.0.1 makes both sources share ${CPM_SOURCE_CACHE}/eigen/5.0.1. Both have tag 5.0.1 at the same commit, bc3b398.

Impacts:

• Existing CPM caches clone Eigen once more into the new tag-keyed directory.
• When GitLab can't be reached, configure prints a CMake warning instead of failing.
• Not covered: if GitLab answers the check but the clone itself fails partway, the build still fails.

Also documents the branch naming convention (^(feature|bugfix|fix|refactor|docs|chore|test|release)/.+, except main, v[0-9.]*, and gh-pages) in CLAUDE.md.

Type of change

• Enhancement (non-breaking change which improves existing functionality)

How Has This Been Tested?

I configured a minimal project that includes the recipe, using an empty CPM_SOURCE_CACHE. To simulate an outage, I redirected gitlab.com to a dead address through git's url.<base>.insteadOf setting.

• GitLab reachable: clones from GitLab at bc3b398, no warning.
• GitLab blocked: warns and clones bc3b398 from the GitHub mirror.
• GitLab blocked, Eigen already cached: no check, no clone, configure succeeds.
• Full project configure (-DIPC_TOOLKIT_BUILD_TESTS=ON) succeeds with the new cache layout.

Test Configuration:

• OS and Version: macOS 26.6.2
• CMake: 4.1.1

Checklist

• I have followed the project style guide
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.98%. Comparing base (7017fa9) to head (7599c40).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #277   +/-   ##
=======================================
  Coverage   96.97%   96.98%           
=======================================
  Files         221      221           
  Lines       22426    22426           
  Branches     1540     1539    -1     
=======================================
+ Hits        21747    21749    +2     
+ Misses        679      677    -2     

Flag	Coverage Δ
unittests	96.98% <ø> (+<0.01%)	⬆️

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