Add Support for Nonmanifold Smooth Edges

[zfergus]

Description

Refactor edge-to-faces mapping and generalize edge3 primitive to N-face neighbors

• Change edges_to_faces from fixed-size matrix to vector of vectors
• Update Edge3 to support arbitrary number of adjacent faces
• Generalize smooth_edge3_term and derivatives for N-face neighbors
• Increase N_EDGE_NEIGHBORS_3D from 4 to 6
• Update Point3 and tests to use Eigen::MatrixX3d for tangent/neighbor arrays

Type of change

• Enhancement (non-breaking change which improves existing functionality)

[codecov]

Codecov Report

❌ Patch coverage is 98.39744% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.75%. Comparing base (f313470) to head (73cac61).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/ipc/smooth_contact/primitives/edge3.cpp	98.37%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #223      +/-   ##
==========================================
- Coverage   97.20%   95.75%   -1.46%     
==========================================
  Files         160      159       -1     
  Lines       24926    16434    -8492     
  Branches      899      913      +14     
==========================================
- Hits        24229    15736    -8493     
- Misses        697      698       +1     

Flag	Coverage Δ
unittests	95.75% <98.39%> (-1.46%)	⬇️

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