feat: add seasonal state-space branch to Gibbs sampler

- sampler.rs: Add seasonal branch in run_single_chain_static() with
  local_level_seasonal_smoother, sample_sigma2_seasonal (InvGamma
  conjugate posterior), sample_sigma2_obs_seasonal, and post-period
  seasonal state propagation. Add 10 unit tests for sigma2_seasonal.
  Remove dead code (scale_matrix, invert_matrix) from rust-speedup merge.
- state_space.rs: Store SeasonalConfig, add has_seasonal(),
  seasonal_nseasons(), seasonal_duration(). Skip dummy regressors when
  state-space seasonal is active.
- lib.rs: Add sigma_seasonal field to GibbsSamples PyO3 binding.

Author: YuminosukeSato

src/lib.rs

@@ -16,6 +16,8 @@ pub struct GibbsSamples {
     #[pyo3(get)]
     pub sigma_level: Vec<f64>,
     #[pyo3(get)]
+    pub sigma_seasonal: Vec<f64>,
+    #[pyo3(get)]
     pub beta: Vec<Vec<f64>>,
     #[pyo3(get)]
     pub gamma: Vec<Vec<bool>>,
@@ -90,6 +92,7 @@ fn run_gibbs_sampler(
         states: result.states,
         sigma_obs: result.sigma_obs,
         sigma_level: result.sigma_level,
+        sigma_seasonal: result.sigma_seasonal,
         beta: result.beta,
         gamma: result.gamma,
         predictions: result.predictions,