Draft: integrate focused Akinci surface tension PRs

[svchb]

Purpose

Integration view for the complete focused Akinci implementation stack and base branch for the validation work. This draft does not replace review and merge of the individual focused PRs.

Included Stack

• Harden surface tension model configuration #1258 surface-tension configuration hardening
• Clarify and visualize SPH surface normals #1279 surface-normal scoping, explanation, and visualization
• Implement multicolor surface normals and threshold handling #1280 multicolor gradients and threshold handling
• Add dimensionally consistent 2D Akinci kernels #1259 dimensionally consistent 2D Akinci kernels
• Fix Akinci free-surface correction with continuity density #1261 required WCSPH and EDAC free-surface correction support
• Use conservative Akinci pair forces across resolutions #1266 conservative adaptive-resolution Akinci pair forces
• Add Akinci surface tension support to IISPH #1267 Akinci surface tension and correction support in IISPH

The experimental physical-coefficient model from #1260 is intentionally excluded. Multiphase pair coefficients and pair reference densities remain dedicated future work rather than being represented by a generic average. The aggregate head is identical to #1267; targeting main provides the complete implementation diff.

Follow-Up

• Add Akinci model shootout with IISPH support #1263 adds the parameterized WCSPH/EDAC/IISPH cube-to-sphere shootout and resolution study.
• Surface Tension Prototype V1  #1253 remains open as the source umbrella while the focused stack is reviewed.

Verification

• complete unit test target passed
• aggregate surface-tension tests: 182/182
• IISPH system tests: 155/155
• reduced validation matrix and all four plotting scripts passed on Add Akinci model shootout with IISPH support #1263
• JuliaFormatter 2.1.1 and every per-layer git diff --check passed

Stack Order

Focused merge order is #1258, #1279, #1280, #1259, #1261, #1266, then #1267. Supporting views are #1262, #1263, and historical umbrella #1253.

All focused PRs target upstream main because GitHub cannot use contributor-fork branches as upstream PR bases. Each PR links a fork comparison that isolates its own review layer.

[codecov]

Codecov Report

❌ Patch coverage is 96.27507% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.62%. Comparing base (a720b1d) to head (0e83e06).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/preprocessing/particle_packing/system.jl	50.00%	3 Missing ⚠️
src/schemes/fluid/surface_normal_sph.jl	92.68%	3 Missing ⚠️
src/schemes/fluid/surface_tension.jl	94.91%	3 Missing ⚠️
src/general/semidiscretization.jl	81.81%	2 Missing ⚠️
src/schemes/fluid/fluid.jl	75.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1262      +/-   ##
==========================================
+ Coverage   90.39%   90.62%   +0.23%     
==========================================
  Files         138      139       +1     
  Lines       11322    11611     +289     
==========================================
+ Hits        10234    10523     +289     
  Misses       1088     1088              

Flag	Coverage Δ
total	90.63% <96.27%> (+0.23%)	⬆️
unit	79.71% <94.26%> (+2.22%)	⬆️

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