Function sequential_blend_step 

pub fn sequential_blend_step(
    opt_state: &OptimizationState_blend,
    config: &Config,
) -> DVector<f64>

Smart sequential hybrid GEDIIS/GDIIS blend step.

Mimics the phased switching of sequential_hybrid_gediis_step but for blend methods:

• Phase 1: Pure GDIIS when RMS gradient >= switch_rms
• Phase 2: Gradient-weighted blend when RMS gradient < switch_rms AND RMS displacement > switch_step
• Phase 3: Pure GDIIS when RMS displacement <= switch_step

Arguments

• opt_state - Experiment optimization state.
• config - Configuration including phase thresholds.

Returns

The new geometry from the selected phase.