Add pure-Python result post-processing helpers

[modelsbridgeaicom-ship-it]

Fixes #57

Summary:

• add pure-Python result helpers for counts, bitstring relabeling, marginals, and Z-parity expectation values
• document the little-endian qubit convention used by the helpers
• re-export the helpers from the package root and update the package-init template
• add Bell-state, rounding, validation, and re-export tests with 100% branch coverage

AI disclosure:

• This PR was prepared with AI assistance. The validation commands below were run locally before submission.

Validation:

• PYTHONUTF8=1 uv run pytest
• uv run ruff check
• uv run ruff format --check
• uv run ty check ionq_core/

[splch]

Thanks - the strongest input validation of the field. Closing in favor of another submission.