Add option to allow/disallow extrapolation in PointCollisionData.

[jorgensd]

Addresses #4450.

Makes it optional to perform extrapolation within cell bounding box for closest point detection (and in turn non-matching interpolation).

Default behavior of allowing extrapolation is kept to avoid breaking user code.

C++ changes have been co-authored with Claude. Test is based on code from @MathPhy42. I have reviewed the contribution.

Other fixes

• Buffer-hoisting: In two places in determine_point_ownership there were arrays containing local coordinate_dofs geometry data being made in hot (and very hot) loops. Resolved by pre-allocating a buffer and adding scratch memory to compute_first_colliding_cell.
• Improved documentation: More explicit documentation as requested by reviewers
• Subtle bug fix a dist>0 is replaced by dist>=0. Very unlikely to trigger with GJK, as we rarely get machine 0, but better than not having it.

[garth-wells]

It would be good to run an LLM over the docstrings for the affected functions. They can be made easier to read.

[jorgensd]

Further fixes made by Claude and me:

• Sentinel bug: current_dist > 0 → current_dist >= 0 in the closest-cell aggregation loop. -1 is the "not computed" sentinel; a genuinely closest candidate can legitimately have squared distance 0 (point exactly on a cell boundary), and the old check rejected that valid answer the same way it rejected the sentinel.
• Buffer hoisting: compute_first_colliding_cell now takes a coordinate_dofs scratch-buffer span (sized to fit, oversized buffers explicitly allowed for future mixed-topology support) instead of allocating internally. determine_point_ownership hoists one buffer before its point loop and reuses it for both the initial collision loop and the closest-cell fallback loop (previously only the fallback loop had been hoisted).
• Added a note documenting that find_closest_cell must be uniform across all ranks of mesh.comm(), since it gates collective MPI calls.
• Renamed send_extrapolate/dest_extrapolate → send_needs_closest_cell/dest_needs_closest_cell for consistency with the renamed public parameter.
  Added missing direct includes: , , , .
• Replaced auto with explicit types on plain std::int32_t/T values (loop variables over collisions.links(...), and current_dist/d in the aggregation loop), per AGENTS.md.