Make continuity_equation! and density_diffusion! work on Ref values

[efaulhaber]

This PR makes continuity_equation! and density_diffusion! work on Ref values instead of directly writing to dv. Together with #1116, it makes it possible to accumulate drho_particle over all neighbors and only once write it to dv.
It is part of #1131.

[codecov]

Codecov Report

❌ Patch coverage is 75.36232% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 67.17%. Comparing base (419d5b9) to head (6bc3034).

Files with missing lines	Patch %	Lines
src/schemes/fluid/shifting_techniques.jl	18.18%	9 Missing ⚠️
src/schemes/boundary/open_boundary/rhs.jl	0.00%	6 Missing ⚠️
...fluid/weakly_compressible_sph/density_diffusion.jl	50.00%	2 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (419d5b9) and HEAD (6bc3034). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (419d5b9)	HEAD (6bc3034)
total	1	0

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1130       +/-   ##
===========================================
- Coverage   89.00%   67.17%   -21.83%     
===========================================
  Files         128      128               
  Lines        9838     9846        +8     
===========================================
- Hits         8756     6614     -2142     
- Misses       1082     3232     +2150     

Flag	Coverage Δ
total	?
unit	67.17% <75.36%> (+0.05%)	⬆️

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[Copilot]

Pull request overview

This PR refactors density-rate updates so continuity_equation! and density_diffusion! can accumulate into a Ref (e.g., drho_particle) rather than writing directly into dv, enabling a later optimization to write dv once per particle (as described in #1116 / #1131).

Changes:

• Update fluid WCSPH and EDAC RHS interaction loops to pass v_a/v_b and accumulate drho_particle::Ref via continuity_equation!, then write via write_drho_particle!.
• Refactor density_diffusion! to accumulate into drho_particle::Ref and adjust call sites accordingly.
• Adjust shifting-technique helpers to operate on/return a transformed v_diff value.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/schemes/structure/structure.jl	Switch structure–fluid continuity update to accumulate into Ref and add a write_drho_particle! writeback path.
src/schemes/fluid/weakly_compressible_sph/rhs.jl	Accumulate density-rate in Ref and write once via write_drho_particle!.
src/schemes/fluid/weakly_compressible_sph/density_diffusion.jl	Make density diffusion accumulate into Ref instead of writing to dv.
src/schemes/fluid/shifting_techniques.jl	Update shifting continuity helpers to accept/return v_diff directly.
src/schemes/fluid/fluid.jl	Refactor continuity_equation! to write into Ref and introduce write_drho_particle! dispatch.
src/schemes/fluid/entropically_damped_sph/rhs.jl	Use Ref accumulation for continuity equation and defer writeback.
src/schemes/boundary/wall_boundary/rhs.jl	Use Ref accumulation for boundary dummy-particle continuity update.
src/schemes/boundary/open_boundary/rhs.jl	Update density diffusion call to use Ref accumulation (continuity term remains direct).

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