Implement sorting of neighbor lists for PrecomputedNeighborhoodSearch

[efaulhaber]

This PR implements the "sorted NL" optimization mentioned in trixi-framework/TrixiParticles.jl#968. See this PR for benchmark results.

[Copilot]

Pull request overview

This PR implements sorted neighbor lists for PrecomputedNeighborhoodSearch to improve cache performance on CPUs and coalesced memory access on GPUs. The optimization involves sorting neighbor lists after construction, which can be controlled via a new sort_neighbor_lists parameter.

Key Changes:

• Added sorteach! function to sort each inner vector in DynamicVectorOfVectors and Vector{Vector{T}}
• Integrated sorting into PrecomputedNeighborhoodSearch with a new configurable sort_neighbor_lists field (default true)
• Reorganized test suite into more structured testsets and added comprehensive tests for the new sorting functionality

Reviewed changes

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

Show a summary per file

File	Description
src/vector_of_vectors.jl	Implements sorteach! for sorting inner vectors; adds default_backend method for DynamicVectorOfVectors
src/nhs_precomputed.jl	Adds sort_neighbor_lists parameter to constructor and integrates sorting into neighbor list initialization
src/util.jl	Moves DynamicVectorOfVectors default_backend method to vector_of_vectors.jl; adds method for PermutedDimsArray
src/PointNeighbors.jl	Reorders includes to place util.jl before vector_of_vectors.jl to resolve dependencies
test/vector_of_vectors.jl	Reorganizes tests into testsets and adds new tests for sorteach! functionality

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[codecov]

Codecov Report

❌ Patch coverage is 77.77778% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.75%. Comparing base (68b4608) to head (9838285).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/vector_of_vectors.jl	80.00%	3 Missing ⚠️
src/util.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #138      +/-   ##
==========================================
- Coverage   84.95%   84.75%   -0.21%     
==========================================
  Files          15       15              
  Lines         698      715      +17     
==========================================
+ Hits          593      606      +13     
- Misses        105      109       +4     

Flag	Coverage Δ
unit	84.75% <77.77%> (-0.21%)	⬇️

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