add difference of qubes

[mathleur]

Description

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

[mathleur]

@copilot can you review this PR?

[Copilot]

@copilot can you review this PR?

I reviewed the PR changes: no material bugs, logic errors, or security issues found in the new difference implementation. I also ran the Rust targeted tests (cargo test -p qubed --test test_difference) and all 19 tests passed. I couldn’t run the Python test file locally in this environment because pytest is not installed.