Harden surface tension model configuration

[svchb]

Summary

• validate surface-tension coefficients as finite, non-negative real values
• avoid allocating and computing surface normals for cohesion-only Akinci models
• stabilize Akinci adhesion evaluation near the compact-support boundary
• promote mixed surface-normal threshold types
• avoid a capillary time-step restriction for zero Morris coefficients
• document surface-tension model requirements

Scope

This is the independent root of the focused Akinci stack extracted from draft #1253.

The current focused sequence is #1258, #1279, #1280, #1259, #1261, #1266, and #1267. PR #1262 is the aggregate implementation view, and #1263 contains validation only.

Verification

• review layer: 9 files, 226 additions, 44 deletions
• focused surface-tension tests: 82/82
• complete unit test target passed with eight Julia threads
• JuliaFormatter 2.1.1 passed

Stack Order

Focused merge order:

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

Supporting views:

• Draft: integrate focused Akinci surface tension PRs #1262 - aggregate implementation view
• Add Akinci model shootout with IISPH support #1263 - validation and resolution study
• Surface Tension Prototype V1  #1253 - historical umbrella

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 94.54545% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.49%. Comparing base (a720b1d) to head (cc239f4).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/schemes/fluid/surface_tension.jl	89.65%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1258      +/-   ##
==========================================
+ Coverage   90.39%   90.49%   +0.10%     
==========================================
  Files         138      139       +1     
  Lines       11322    11358      +36     
==========================================
+ Hits        10234    10278      +44     
+ Misses       1088     1080       -8     

Flag	Coverage Δ
total	90.49% <94.54%> (+0.10%)	⬆️
unit	77.63% <89.09%> (+0.15%)	⬆️

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