Improve GPU performance of fluid-* interact

[efaulhaber]

This PR rewrites the interact! function for fluid-* interaction and is part of #1131.

• The foreach_point_neighbor loop is now unrolled into an @threaded for particle and foreach_neighbor.
• With this change, I can now load data for particle a once instead of loading it again for each neighbor.
• This unrolled loop now also allows me to accumulate the dv values over all neighbors and write to dv only once per particle.

[codecov]

Codecov Report

❌ Patch coverage is 94.73684% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.06%. Comparing base (6ee0bf4) to head (e7369d0).

Files with missing lines	Patch %	Lines
src/general/neighborhood_search.jl	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1116      +/-   ##
==========================================
- Coverage   89.13%   89.06%   -0.08%     
==========================================
  Files         128      128              
  Lines        9869     9869              
==========================================
- Hits         8797     8790       -7     
- Misses       1072     1079       +7     

Flag	Coverage Δ
total	89.06% <94.73%> (-0.08%)	⬇️
unit	67.26% <81.57%> (+0.01%)	⬆️

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[Copilot]

Pull request overview

Refactors the fluid-* interact! implementation to improve GPU performance by unrolling the point-neighbor iteration into a per-particle threaded loop and reducing repeated loads/writes.

Changes:

• Rewrites WCSPH interact! to thread over particles, use foreach_neighbor, and accumulate per-particle RHS contributions before writing.
• Introduces foreach_neighbor wrapper with a GPU-unsafe fast path (bounds-check-free) for neighbor traversal.
• Adds a 3D WCSPH ContinuityDensity fast path that combines velocity+density loads using SIMD wide loads; updates correction and pressure helper APIs accordingly.

Reviewed changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/schemes/fluid/weakly_compressible_sph/rhs.jl	Main WCSPH interact! refactor; adds neighbor_pressure and velocity_and_density helpers + SIMD load fast path.
src/schemes/fluid/implicit_incompressible_sph/rhs.jl	Switches to neighbor_pressure helper to avoid redundant pressure loads / enable mirroring behavior.
src/general/neighborhood_search.jl	Adds foreach_neighbor wrapper with a GPU specialization calling PointNeighbors.foreach_neighbor_unsafe.
src/general/corrections.jl	Adjusts free_surface_correction API to accept (rho_a, rho_b) and compute rho_mean internally.
src/TrixiParticles.jl	Adds SIMD import required by the new 3D fast path.
Project.toml	Adds SIMD dependency and bumps PointNeighbors compat to 0.6.6.

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[efaulhaber]

/run-gpu-tests

[efaulhaber]

/run-gpu-tests