Implement quantum noise (gate depolarizing) in Backend A simulation

[q-inho]

Add an optional quantum noise feature to the Backend A simulator so traces can reflect realistic hardware error instead of ideal evolution. Start with a simple depolarizing model that applies a small random Pauli error after each unitary gate (configurable probabilities, deterministic via seed).
Keep noise disabled by default and restrict it to mode="preview" so validation remains strictly noiseless.