• Avoid warning about coloring in plot_trajectories
• Fix: forward eigvec_key to fallback method in early_cell()
• Performance optimizations (thanks to @svdrecbd for PR #176):
  • Faster max-min waypoint sampling: O(N·K²) → O(N·K)
  • PCA slicing optimization to avoid redundant recomputation
  • Sparse LU solver for absorption probabilities
  • Vectorized perspective matrix and Dijkstra shortest paths
  • Vectorized Markov chain construction and terminal state detection
  • Optional sklearn backend for diffusion kernel (kernel_backend parameter)
• Fix: run_pca now correctly preserves existing X_pca when using custom pca_key