Improve GPU performance with shifting

[efaulhaber]

Here is a benchmark of interact! on an A4500 with FP32. I ran the periodic_channel_2d.jl example with 125k particles and benchmarked with:

@b TrixiParticles.interact!($(copy(ode.u0.x[1])), $(ode.u0.x)..., $(ode.p.systems[1]), $(ode.p.systems[1]), $(ode.p)) seconds=1

Setup	Time
No shifting	297.812 μs
main with ConsistentShiftingSun2019	3.117 ms
inlining continuity_equation_shifting!	431.567 μs
@propagate_inbounds	360.704 μs
vdiff_shifting instead of continuity_equation_shifting!	347.360 μs

[Copilot]

Pull Request Overview

This PR improves GPU performance for particle shifting operations by refactoring from mutating to non-mutating functions and adding @propagate_inbounds annotations. The benchmark shows a ~9x performance improvement (from 3.117 ms to 347.360 μs) for ConsistentShiftingSun2019 on an A4500 GPU.

Key changes:

• Refactored continuity_equation_shifting! to non-mutating continuity_equation_shifting_term that returns values instead of mutating arrays
• Added @propagate_inbounds annotations to performance-critical functions
• Reorganized timing instrumentation for better profiling granularity

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/schemes/fluid/shifting_techniques.jl	Refactored continuity equation shifting from mutating to non-mutating pattern, added @propagate_inbounds annotations, and split timing blocks
src/schemes/fluid/fluid.jl	Integrated non-mutating continuity_equation_shifting_term into continuity equation calculation
src/schemes/boundary/open_boundary/system.jl	Added timing instrumentation wrapper around open boundary update
src/preprocessing/particle_packing/system.jl	Added timing instrumentation for particle packing operations
src/callbacks/update.jl	Removed redundant timing wrappers (moved to individual functions)

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[codecov]

Codecov Report

❌ Patch coverage is 7.50000% with 37 lines in your changes missing coverage. Please review.
✅ Project coverage is 64.83%. Comparing base (4725885) to head (ef9d65d).

Files with missing lines	Patch %	Lines
src/schemes/fluid/shifting_techniques.jl	10.00%	18 Missing ⚠️
src/preprocessing/particle_packing/system.jl	0.00%	8 Missing ⚠️
src/schemes/boundary/open_boundary/system.jl	0.00%	7 Missing ⚠️
src/callbacks/update.jl	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #974      +/-   ##
==========================================
- Coverage   64.84%   64.83%   -0.01%     
==========================================
  Files         120      120              
  Lines        8565     8566       +1     
==========================================
  Hits         5554     5554              
- Misses       3011     3012       +1     

Flag	Coverage Δ
unit	64.83% <7.50%> (-0.01%)	⬇️

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