Add Quantinuum Nexus backend

[jf-kong]

Summary

• New NexusClientBackend (nexus-client) that submits Qibo circuits through Quantinuum Nexus using the latest qnexus APIs.
• Two execution paths in one backend, selected by the platform argument:
  • H-Series (hseries:H2-1LE, hseries:H2-1, …): qibo → OpenQASM → pytket → Nexus compile+execute, with H2 syntax-checker HQC estimation.
  • Helios (helios:Helios-1, helios:Helios-1E): qibo → pytket → guppylang HUGR package → qnx.start_execute_job, with qnx.hugr.cost_confidence for HQC estimation.
• Batch execution (execute_circuits / estimate_circuits) on both paths.
• Helios cost estimation always targets Helios-1SC, since qnx.hugr.cost_confidence builds its costing job from QuantinuumConfig(device_name=f"{system_name}SC") and only Helios-1SC exists as a syntax checker.
• Optional install: pip install qibo-cloud-backends[nexus] (pulls qnexus, pytket, guppylang).
• Lazy imports throughout: importing qibo_cloud_backends does not require the nexus extras; auth and project resolution are deferred until the first execute / estimate call.

What changed

• src/qibo_cloud_backends/nexus_client.py — NexusClientBackend (Qibo NumpyBackend subclass) implementing execute_circuit, execute_circuits, estimate_circuit, estimate_circuits.
• src/qibo_cloud_backends/nexus_config.py — NexusBackendConfig dataclass, parse_platform, build_nexus_backend_config (handles hseries, helios, aer; splits options between HeliosConfig and HeliosEmulatorConfig).
• src/qibo_cloud_backends/nexus_helios.py — Helios HUGR package builder (pytket → AutoRebase to {CX, H, Rz} → guppy load_pytket → generated entrypoint with result(...) / discard(...)) and map_helios_result_to_qibo for converting Helios shot results into a Qibo MeasurementOutcomes.
• src/qibo_cloud_backends/nexus_translation.py — qibo → pytket via OpenQASM 2.0, with normalization for measurement-register names, multi-control decomposition, and Y-basis rotation rebuild
• src/qibo_cloud_backends/nexus_results.py — generic count → Qibo bitstring / MeasurementOutcomes mapping (shared by H-Series and Helios result paths).
• src/qibo_cloud_backends/nexus_auth.py, nexus_errors.py — auth / project plumbing and the typed exception hierarchy (NexusBackendError, NexusAuthError, NexusResultMappingError, UnsupportedExecutionError).
• src/qibo_cloud_backends/__init__.py — register nexus-client in MetaBackend.load / CLIENTS / _client_available; lazy class export via __getattr__.
• pyproject.toml — adds qnexus, pytket, guppylang.
• README.md, doc/source/index.rst, doc/source/backends.rst — install instructions and quickstart for the Nexus backend.

Test plan

50 nexus-specific tests, all passing locally:

• tests/test_translator.py (8) — qibo → pytket round-trip, multicontrol decomposition, Helios measurement stripping.
• tests/test_config.py (13) — platform parsing, HeliosConfig / HeliosEmulatorConfig resolution, options split (n_qubits sizing on emulator).
• tests/test_results.py (7) — count normalization, endianness, MeasurementOutcomes mapping.
• tests/test_helios.py (3) — HUGR entrypoint generation (measurement order, discard for unmeasured qubits), pytket pre-Guppy rebase.
• tests/test_backend_contract.py (16) — backend contract: H-Series and Helios paths for execute_circuit / estimate_circuit / execute_circuits, including the regression test that locks in (a) per-program max_cost flowing through qnx.start_execute_job, (b) emulator state sized to the widest circuit's nqubits, (c) no attempt_batching injection, (d) cost estimation targeting Helios-1SC even when the user platform is Helios-1E.
• tests/test_nexus_meta_backend.py (3) — MetaBackend.load(client="nexus-client", ...), default-platform fallthrough, set_backend("qibo-cloud-backends", client="nexus-client", ...). Split out from tests/test_backends.py so they collect without the optional braket / qiskit extras installed.

Notes

• All Nexus authentication is lazy (_ensure_connected); just importing the package never triggers a Nexus login or network call.
• The qnexus 0.x API surface is moving; the version pin is intentionally >=0.39,<1.

[scarrazza]

Closing this in favour of qiboteam/nexus-backend#1 (review)