#542 preserve _id, _time, _version, _job on Fbe.delete recreate path

[edmoffo]

Issue #542 reports that Fbe.delete reassigns the four system properties whenever the factbase is decorated by Fbe.fb. The destroy-and-recreate cycle in lib/fbe/delete.rb deleted _id from the snapshot up front, then routed the recreated fact through the Factbase::Pre block that assigns _id, _time, _version, and _job on insert, and the restoration loop's next unless c[k].nil? guard refused to overwrite the Pre-assigned values. Any external reference to the original _id, including follower facts and later (eq _id N) lookups, was left dangling.

The fix strips the four system properties from the new fact's internal map after Pre fires but before the restoration loop runs, so the loop carries the original values forward. before.delete(id) is gone, so _id survives into the snapshot like the rest. test_deletes_safely continues to pass because its single-fact factbase still produces the same _id value, while a new test_preserves_system_props_on_decorated_fb pins the contract against a two-fact factbase where the Pre arithmetic would otherwise drift.

Verification on this branch: bundle exec ruby -Ilib -Itest test/fbe/test_delete.rb reports 10 tests, 21 assertions, 0 failures, 0 errors, 0 skips; rubocop lib/fbe/delete.rb test/fbe/test_delete.rb reports 0 offenses. Reverting only the lib/fbe/delete.rb change flips the new test to Expected: 1, Actual: 3, confirming it catches the bug.

Closes #542

[0crat]

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