Clarify and visualize SPH surface normals

[svchb]

Summary

• scope surface normals as general interface-geometric data rather than a surface-tension-only concept
• allow surface-normal methods to be configured independently for analysis and VTK output
• distinguish raw color gradients from unit normals used by Morris formulations
• explain color-field transitions, orientation, normalization, and numerical filtering
• add generated CairoMakie figures for a diffuse color-field profile and two-dimensional interface normals
• scope color sanity checks to ColorfieldSurfaceNormal

Stack

Depends on #1258. Fork branches cannot be selected as bases for pull requests into the upstream repository, so this PR targets main and temporarily shows the preceding commit.

Review-only diff: svchb/TrixiParticles.jlOpen@surface-tension/akinci-core-hardening...surface-normals/interface-documentation

Verification

• review layer: 7 files, 138 additions, 31 deletions
• both CairoMakie figures rendered successfully with CairoMakie 0.15
• targeted color-validation checks passed
• 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 94.73684% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.50%. Comparing base (a720b1d) to head (02d1689).
⚠️ 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    #1279      +/-   ##
==========================================
+ Coverage   90.39%   90.50%   +0.10%     
==========================================
  Files         138      139       +1     
  Lines       11322    11358      +36     
==========================================
+ Hits        10234    10279      +45     
+ Misses       1088     1079       -9     

Flag	Coverage Δ
total	90.50% <94.73%> (+0.10%)	⬆️
unit	77.64% <89.47%> (+0.15%)	⬆️

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[svchb]

Coordination note: #1282 now contains the generalized surface-method API, standalone configuration, common documentation, and output plumbing that overlap this draft. After #1282 settles, this PR should be reduced to any still-unique explanation/figures, or closed if those pieces are fully subsumed. No replacement stack layer is planned.