Add gitlab-ci-test-base module for test standardization

Summary

Adds a new gitlab-ci-test-base flake-parts module that provides common test infrastructure, eliminating duplication across all 15 test flakes.

Changes

• New module: nix/modules/flake/gitlab-ci-test-base.nix

  • Automatically imports gitlab-ci.flakeModule and treefmt-nix.flakeModule
  • Sets default system to x86_64-linux
  • Enables gitlab.ci only on x86_64-linux (prevents duplicate jobs)
  • Enables flake validation by default (flake-show, flake-lock, strict-inputs)
  • Sets flake.flakes = [ "." ] for validator modules

• Test standardization: All 15 tests converted to use test-base module

  • Tests now use single nix/ci.nix file
  • Removed 24 redundant files (configuration.nix, default.nix, systems.nix, treefmt.nix)
  • Reduces test configuration duplication by 60%+
  • Net reduction: 285 deletions → 123 insertions

• Documentation:

  • Updated ChangeLog.md for 0.18.0 release
  • Added "Testing and Development" section to README.md
  • Documented test-base module usage and benefits

Benefits

✅ Consistent test structure across all tests ✅ Automatic flake validation for all tests ✅ Significantly reduced code duplication ✅ Easier to write new tests ✅ All tests now validate their own flake structure

Test Plan

• All 15 tests generate valid CI output with flake validation jobs
• Main CI pipeline includes all tests in matrix
• All flake.lock files updated

🤖 Generated with Claude Code