Represents an arbitrary spin-1/2 state as a spinor $(\alpha, \beta)^T$. Methods: - `from_angles(theta, phi)` — Create state from Bloch sphere angles. - `measure(axis)` — Return probabilities for spin-up and spin-down along a given axis. - `bloch_vector()` — Return the Bloch vector $(n_x, n_y, n_z)$.