fix #26: Add StatevectorSampler backend for running quantum circuits

[at264939-ctrl]

Summary

Implements a pure-Rust statevector-based sampler that allows users to execute QuantumCircuit objects locally and obtain shot-based measurement results. This addresses the lack of any backend/simulation capability in qiskit-rs.

Design

• No additional dependencies: uses a built-in xorshift64 PRNG instead of pulling in rand.
• No C API dependency for simulation: the Qiskit C API does not currently expose simulation primitives, so this implements the linear algebra directly in Rust.
• The sampler walks circuit.instructions(), applies the corresponding unitary matrices to a dense statevector of size 2^n, then samples outcomes from the Born-rule probability distribution.

What's included

• src/sampler.rs — StatevectorSampler, SamplerResult, gate matrices for all 25+ gates exposed by QuantumCircuit
• src/lib.rs — re-exports StatevectorSampler and SamplerResult
• tests/test_sampler.rs — integration tests (Bell state, GHZ-10, X gate, Hadamard statistics, RZ phase)

Usage

use qiskit_rs::{QuantumCircuit, StatevectorSampler};

let mut qc = QuantumCircuit::new(2, 2);
qc.h(0);
qc.cx(0, 1);
qc.measure(0, 0);
qc.measure(1, 1);

let sampler = StatevectorSampler::new();
let result = sampler.run(&mut qc, 1024);
println!("{:?}", result.counts());
// {"00": 507, "11": 517}

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