Add functions mapreduce_neighbor and mapreduce_neighbor_unsafe

[efaulhaber]

This PR allows us to convert the awkward

dv_particle = Ref(zero(v_a))

foreach_neighbor(...) do particle, neighbor, pos_diff, distance
    ...
    dv_particle[] += result
end

to a cleaner

dv_particle = mapreduce_neighbor(+, ..., init=zero(v_a)) do particle, neighbor, pos_diff, distance
    ...
    return result
end

in trixi-framework/TrixiParticles.jl#1211.

This is necessary for vectorization on the CPU.

[codecov]

Codecov Report

❌ Patch coverage is 95.74468% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.38%. Comparing base (f25b0ab) to head (110aef6).

Files with missing lines	Patch %	Lines
test/neighborhood_search.jl	88.88%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #158      +/-   ##
==========================================
+ Coverage   86.06%   86.38%   +0.31%     
==========================================
  Files          15       15              
  Lines         739      771      +32     
==========================================
+ Hits          636      666      +30     
- Misses        103      105       +2     

Flag	Coverage Δ
unit	86.38% <95.74%> (+0.31%)	⬆️

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[Copilot]

Pull request overview

This PR introduces mapreduce_neighbor / mapreduce_neighbor_unsafe as new public APIs to compute per-point reductions over neighbors, and refactors the existing neighbor-iteration internals so foreach_neighbor is implemented as a special case of the new map-reduce kernel.

Changes:

• Add mapreduce_neighbor and mapreduce_neighbor_unsafe to the public API (exports) and implement them for all neighborhood-search backends.
• Refactor backend-specific neighbor loops (GridNeighborhoodSearch, PrecomputedNeighborhoodSearch, and the generic fallback) to return a reduction value (mapreduce_neighbor_inner) and adapt foreach_neighbor to use it.
• Update tests to use push! for neighbor collection and add allocation tests for foreach_neighbor / mapreduce_neighbor plus keyword-argument error checks.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
test/neighborhood_search.jl	Fix neighbor list construction (push!) and add tests for mapreduce_neighbor(*), plus allocation and missing-init checks.
src/PointNeighbors.jl	Export the new mapreduce_neighbor and mapreduce_neighbor_unsafe APIs.
src/nhs_precomputed.jl	Implement reduction-returning neighbor loop for PrecomputedNeighborhoodSearch.
src/nhs_grid.jl	Implement reduction-returning neighbor loop for GridNeighborhoodSearch.
src/neighborhood_search.jl	Add the new APIs and refactor generic/unsafe neighbor iteration to use the map-reduce inner loop.

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