feat: Phase 2 (fragment migration), Phase 3 (testing infrastructure), Phase 4 (CI/CD)

Phase 2: Fragment Migration
- Migrated 42 ingredients to 54 fine-grained fragments
- Replaced %include with %ksappend syntax
- Created organized fragment structure (platform/generic-43, platform/generic-rawhide, shared/)
- Updated generator to use fragment paths
- All 16 manifest variants generate successfully

Phase 3: Testing Infrastructure
- Created containerized test runner (tests/container/)
- Added integration test suite (tests/integration/)
- Created golden master fixtures (tests/fixtures/expected_recipes/)
- 41 tests passing (36 unit + 5 integration)
- 54 fragments, 16 recipes validated

Phase 4: CI/CD Integration
- 5 new workflows: validate-recipes, validate-fragments, test-generation, container-tests, build-iso
- Added validation gates before ISO builds
- Works with existing fedora-runner-image
- Local testing support via act_runner

fragments/shared/section-data/post/base.ks

@@ -0,0 +1,9 @@
+# Core post-installation configuration
+
+%post --log=/mnt/sysimage/root/post.log # Beginning of the post-installation section. Log all messages to a given file
+
+localectl set-keymap ch-fr # Set keymap to `ch-fr`. Alternatively, `us` can be picked.
+dnf update -y # Update the system
+grub2-mkconfig -o /boot/grub2/grub.cfg # Unsure it is actually useful
+
+%end # End of the %post section

fragments/shared/section-data/post/nochroot.ks

@@ -0,0 +1,5 @@
+# Post-installation no-chroot section
+
+%post --nochroot --log=/mnt/sysimage/root/post-nochroot.log # Beginning of the post-installation section. Log all messages to a given file
+
+%end # End of the %post section