Add pure-Python results post-processing helpers (counts, bitstrings, marginals, expectation values)

[splch]

Background

The results endpoints (get_job_probabilities, get_variant_probabilities, get_variant_histogram) return IonQ's register-keyed probability mapping (state key → probability) as-is via probs.additional_properties. There are no helpers for the post-processing callers keep rewriting — counts, bitstring relabeling, marginals, expectation values. A small pure-Python module would remove that duplication and give the wrappers (qiskit-ionq, cirq-ionq, pennylane-ionq) a common base.

What to build

A new hand-written module ionq_core/results.py, pure-Python and NumPy-free (like ionq_core/gates.py), with helpers over the probabilities mapping. Suggested set (names open to discussion in the PR):

1. probabilities_to_counts(probabilities, shots) — integer counts summing exactly to shots (largest-remainder rounding).
2. relabel_to_bitstrings(probabilities, num_qubits) — integer state keys → zero-padded bitstrings.
3. marginal(probabilities, qubits, num_qubits) — marginal over a subset of qubits.
4. expectation_z(probabilities, num_qubits) — ⟨Z⊗…⟩ parity, Σ p(x)·(−1)^popcount(x).

Each key is the integer encoding of the measured bitstring (Bell state → {"0": ~0.5, "3": ~0.5}), so int(key) parses it. Capture a real simulator response as a fixture, and document the bit-ordering convention consistently across helpers.

Definition of done

• ionq_core/results.py with the helpers (docstrings + type hints); public names in __all__ and re-exported from ionq_core/__init__.py.
• tests/test_results.py with a Bell-state fixture and rounding edge cases; 100% branch coverage (the repo gates on --cov-fail-under=100).
• SPDX header (AGENTS.md); uv run pytest, uv run ruff check, uv run ruff format --check, and uv run ty check ionq_core/ pass.

Notes

Operate on plain Mapping[str, float] so it works for both the job and variant endpoints and is testable without HTTP. No new runtime deps; no NumPy. References: ionq_core/api/default/get_job_probabilities.py (+ variant endpoints), the pure-Python style in ionq_core/gates.py, and AGENTS.md.