Make dv_shifting, surface_tension_force and adhesion_force inplace

[efaulhaber]

This PR makes dv_shifting, surface_tension_force and adhesion_force inplace and is part of #1131. Instead of accumulating zero SVectors when no shifting/surface tension/adhesion is used, these functions are now no-ops. It also reduces the number of live variables, which reduces register pressure.

Note that these optimizations make almost no difference on modern Hopper GPUs, but are very noticable on older Ampere GPUs.

[codecov]

Codecov Report

❌ Patch coverage is 91.37931% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.70%. Comparing base (155f9f1) to head (20da330).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/schemes/fluid/surface_tension.jl	81.48%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1124      +/-   ##
==========================================
- Coverage   88.71%   88.70%   -0.01%     
==========================================
  Files         128      128              
  Lines        9752     9764      +12     
==========================================
+ Hits         8651     8661      +10     
- Misses       1101     1103       +2     

Flag	Coverage Δ
total	88.70% <91.37%> (-0.01%)	⬇️
unit	66.95% <56.36%> (-0.05%)	⬇️

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[Copilot]

Pull request overview

This PR refactors SPH interaction terms (dv_shifting, surface_tension_force, adhesion_force) to be in-place updates (no-ops when disabled) instead of returning/accumulating zero vectors, aiming to reduce live SSA values and register pressure (notably on older GPUs).

Changes:

• Replaced returned-vector helpers with in-place ...! variants and updated fluid/structure RHS interaction loops to accumulate into a single mutable accumulator.
• Updated surface tension and shifting technique implementations to add their contributions directly into the accumulator (including passing a surface_tension_correction factor in-place).
• Adjusted VTK output computation to use the in-place surface tension force to accumulate per-particle diagnostics.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/schemes/structure/structure.jl	Switches structure–fluid interaction to an in-place adhesion contribution via a mutable accumulator.
src/schemes/structure/rigid_body/system.jl	Converts rigid-body adhesion force helper into an in-place adhesion_force!.
src/schemes/fluid/weakly_compressible_sph/rhs.jl	Updates WCSPH RHS interaction loop to accumulate shifting/surface tension/adhesion contributions in-place.
src/schemes/fluid/surface_tension.jl	Refactors surface tension + boundary adhesion force APIs to ...! and applies correction factor internally.
src/schemes/fluid/shifting_techniques.jl	Refactors shifting momentum-term API to in-place dv_shifting! and updates implementations accordingly.
src/schemes/fluid/entropically_damped_sph/rhs.jl	Updates EDAC RHS interaction loop to use in-place shifting/surface tension/adhesion accumulation.
src/io/write_vtk.jl	Updates surface tension diagnostic accumulation to use the new in-place API.

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[svchb]

/run-gpu-tests

[LasNikas]

/run-gpu-tests