Nmesh section 4 - relaxation mesher internals

[epituch]

Summary

Section 4 of the nmesh migration is implemented on this branch.

• Replaces the OCaml-backed core meshing path with a pure Python relaxation mesher.
• Adds FEM geometry construction from bodies, hints, density rules, and periodic bookkeeping.
• Implements seed preparation for fixed, mobile, simple, boundary, and hint-driven points.
• Ports the iterative relaxation loop with neighbor, shape, volume, and irrelevant-element forces.
• Adds boundary-aware motion constraints, force projection, corner suppression, and convergence checks.
• Adds numba-accelerated neighbor-force kernels for the hot path.
• Builds final topology in Python, including Delaunay simplices, surfaces, links, and mesh assembly.
• Preserves backend integration through the existing mesher entry points.
• Refactors the new mesher into focused modules: geometry, seeding, forces, engine, topology, density, and jit.
• Improves internal readability with docstrings, type hints, and smaller helper functions.
• Adds deterministic integration coverage for fixed-seed, periodic, and hint-driven meshing.
• Expands regression tests for force behavior, boundary handling, projection, and topology filtering.
• Updates project dependencies required by the Python mesher path, including scipy and numba.