feat(executors): add Quantinuum executor

[kpassito]

Summary

Adds a QuantinuumExecutor and QuantinuumExecutorConfig using pytket-quantinuum, registered through ExecutorFactory with provider string "Quantinuum".

This also includes the Circuit.to_pytket() conversion path needed by the executor because the prerequisite converter issue has not merged yet.

Closes #6

Type of change

• Bug fix
• New executor
• Circuit format converter
• Documentation
• Other (describe):

Testing

• I ran pytest tests/ -v and tests pass
• For new executors: tested against local simulator or QVM (describe below)
• For circuit converters: roundtrip test passes with known-correct reference circuit

Test details:

Focused local checks:

• .venv/bin/python -m pytest tests/test_executors.py::TestQuantinuumExecutorConfig tests/test_executors.py::TestQuantinuumExecutor tests/test_executors.py::TestQuantinuumExecutorFactory -q - 9 passed
• .venv/bin/python -m pytest tests/test_circuits.py::TestToPytket -q - 3 passed
• .venv/bin/python -m ruff check marqov/executors/quantinuum.py marqov/executors/factory.py marqov/executors/__init__.py marqov/circuits.py tests/test_executors.py tests/test_circuits.py - passed

The executor tests use a mocked pytket-quantinuum backend, matching the issue's acceptance criteria. I did not run a hardware smoke test.

Checklist

• No hardcoded credentials or API keys
• Handles the canonical gate set from CONTRIBUTING.md §1 (if adding a circuit converter)

For new executors only:

• Registered in ExecutorFactory per CONTRIBUTING.md §3
• get_status() returns device-level availability ("online"/"offline"/"maintenance"), not job-level status

AI disclosure

Codex assisted with implementation, test planning, and local validation for this contribution. The commit messages include the disclosure footer:

Assisted-by: GPT-5 Codex via Codex

[ddri]

Thanks for the PR. I see you're making a number of PRs on projects in the unitaryHACK listing. The concern here would be that AI has done all the work, and there's not been any good-faith effort to really understand the project, make thoughtful decisions, and properly test the solution in context.

To help me understand if that's the case, I'd like to hear your thoughts on this, and talk a little about your experience and approach using the Marqov SDK.