integrate mps state into the backend

[zazabap]

The back end should have multiple options compare to the usual dense state.

---

Note

Introduce a backend abstraction and execute_on_backend API, implement MPS (tensor-network) backend, and add comprehensive dense/MPS tests; bump to 0.2.4.

• Core API:
  • Add states::backend::QuantumStateBackend trait for backend-agnostic execution.
  • Add Circuit::execute_on_backend to run circuits on any backend (e.g., State, MpsState).
  • Helper matrix extractors for 1- and 2-qubit gates (extract_single_qubit_matrix, extract_two_qubit_matrix).
• Backends:
  • Implement QuantumStateBackend for states::State (dense) with matrix-application helpers.
  • Implement QuantumStateBackend for simulators::mps::MpsState with optimized 1/2-qubit ops, SWAP networks for non-adjacent gates, control>target handling, and fallbacks/errors for unsupported 3-qubit/full-matrix cases.
• Exports:
  • Re-export QuantumStateBackend, MpsConfig, MpsState via prelude.
• Tests:
  • Consolidate dense-state circuit tests into tests/test_circuits_dense.rs.
  • Add tests/test_circuits_mps.rs covering MPS execution, dense-vs-MPS parity, and edge cases (e.g., control > target); mark unsupported 3-qubit cases with #[ignore].
• Docs/Version:
  • Bump version to 0.2.4 and update README.md dependency snippet.

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