[ENH] Refactor project_points_onto_surface for robustness and nonlinear projection support

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Refactors the project_points_onto_surface function to enable exact Newton-based projection for linear scalar fields and improve nonlinear field handling with residual re-evaluation requirements. Adds gradient and surface tolerance handling, error conditions for near-zero gradients, and improved validation for scalar and gradient inputs. Updates tests to validate projection correctness, idempotency, and edge cases. Enhances gradient field accessors in dense grid outputs to address inconsistencies.

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• [ENH] Wire finite-fault definitions into fault stack interpolation #93 : 2 dependent PRs (#94 , #96 )
• [ENH] Refactor project_points_onto_surface for robustness and nonlinear projection support #92 👈 (View in Graphite)
• [ENH] Introduce FiniteFault model with validation, serialization, and spline taper support #91
• [FIX] Ensure tensors consistently move to backend device and add tests for GPU-specific behavior #90
• Added a comprehensive technical implementation report for the Multiscale Anisotropic Field Deformation framework. Describes methodology, GPU-accelerated execution via PyKeOps, and dual-criteria octree subdivision for efficient and precise geological model #95
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Merge activity

• Sep 5, 9:11 AM UTC: A user started a stack merge that includes this pull request via Graphite.
• Sep 5, 9:32 AM UTC: Graphite couldn't merge this pull request because a downstack PR [ENH] Introduce FiniteFault model with validation, serialization, and spline taper support #91 failed to merge.