#551: Keep fact replacement atomic

[GHX5T-SOL]

/claim #551

Closes #551.

Summary

• Move the delete step for Fbe.overwrite, Fbe.delete, and Fbe.delete_one inside the same fb.txn block as the replacement insert.
• Add regression tests proving the original fact remains when the replacement insert raises.

Validation

• bundle check
• bundle exec ruby -Itest test/fbe/test_overwrite.rb -- --no-cov
• bundle exec ruby -Itest test/fbe/test_delete.rb -- --no-cov
• bundle exec ruby -Itest test/fbe/test_delete_one.rb -- --no-cov
• git diff --check
• bundle exec rubocop lib/fbe/overwrite.rb lib/fbe/delete.rb lib/fbe/delete_one.rb test/fbe/test_overwrite.rb test/fbe/test_delete.rb test/fbe/test_delete_one.rb
• bundle exec rake test
• bundle exec rubocop
• bundle exec rake

No secrets or live-user data used; validation was local and within the repo scope.

[bibonix]

The delete-then-insert sequence in Fbe.delete, Fbe.delete_one, and both branches of Fbe.overwrite now sits inside the fb.txn block, so a raised insert rolls the delete back instead of leaving the factbase with the old fact gone. The three new regression tests inject a raising Factbase::Pre to force the insert to fail and then assert that the original fact and every untouched property survive the call. CI is green on actionlint, copyrights, markdown-lint, pdd, reuse, typos, xcop, yamllint, and the rake jobs on ubuntu-24.04 and macos-15. No inline comments.

[yegor256]

@GHX5T-SOL merge conflict here

[yegor256]

Please fix the merge conflicts so this pull request can be merged.