[hansen_singleton_1983.md] Update np.random → Generator API

[Chihiro2000GitHub]

Summary

This PR migrates legacy NumPy random API usage in hansen_singleton_1983.md as part of QuantEcon/meta#299.

The only legacy usage was inside simulate_restricted_var, which relied on the global random state via np.random.seed and np.random.multivariate_normal. This is now replaced with a local np.random.default_rng generator, consistent with the other simulation functions in the lecture (starting_values and simulate_multi_asset_nominal_returns) that already use the Generator API.

Details

• Replaced if seed is not None: np.random.seed(seed) with a local rng = np.random.default_rng(seed).
• Replaced np.random.multivariate_normal(...) with rng.multivariate_normal(...).
• rng is defined locally inside simulate_restricted_var, so the function remains self-contained.
• No fixed seed was newly introduced; the existing seed=0 default is preserved, so simulation output is unchanged.
• No prose changes were needed, and there are no Numba (@jit/@njit/@jitclass/parallel/prange) cases in this file.

Note: np.random.default_rng(None) reproduces the original "do not seed" behaviour when seed is None, so the previous if seed is not None guard was dropped without changing semantics.

Hi @mmcky and @HumphreyYang, I'd be grateful if you could take a look when you have time.

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