Add pure-Python result post-processing helpers

[caijintaoduolacmeng-star]

Summary

• Add ionq_core.results with pure-Python helpers for counts, bitstring relabeling, marginals, and all-Z expectation values.
• Re-export the helpers from ionq_core.init and keep the generated package template in sync.
• Add focused branch-coverage tests for Bell-state output, rounding ties, invalid inputs, and qubit-order marginalization.

Closes #57

Verification

• PYTHONUTF8=1 python -m pytest -q -> 251 passed, 28 deselected, 100% coverage
• python -m pytest tests/test_results.py --no-cov -q -> 9 passed

uff check -> All checks passed

uff format --check -> 33 files already formatted

• y check ionq_core/ -> All checks passed
• git diff --check -> passed