Add droplet-coalescence analysis notebook

[drcole17]

Summary

• New notebook at examples/3D_droplet_coalescence/analysis.ipynb that reads silo output via mfc.viz and reports:
  • cells per diameter on each axis
  • interface thickness (0.05 <= alpha_1 <= 0.95) along an axis-aligned ray through each droplet centroid, with a |grad alpha| cross-check
  • midplane sanity slice and an interface-thickness time series
• Works on both an IC-check run (t_step_stop=1) and a full run.

Test plan

• jupyter lab examples/3D_droplet_coalescence/analysis.ipynb from the MFC venv with silo_hdf5/ populated
• Confirm CPD table matches expected (Nx=399 over 4.5D -> ~88.7; Ny=Nz=279 over 3D -> ~93)
• Confirm interface thickness cells agree between the ray method and |grad alpha| cross-check
• Confirm time-series cell is a no-op when only one step is saved

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