Implement multicolor surface normals and threshold handling

[svchb]

Summary

• implement color-weighted gradients across interacting WCSPH, EDAC, and IISPH fluids
• make equal colors suppress an interface and preserve the sign and magnitude of unequal color contrasts
• apply normal validity thresholds consistently to standalone, Akinci, and Morris calculations
• retain raw gradients for standalone/Akinci use and preserve the raw momentum-Morris surface delta before unit normalization
• validate threshold ranges and document their physical interpretation and multiphase limitations
• exclude particle-packing preprocessing through dispatch-based color-field and normal-computation opt-outs
• add multicolor gradient and resulting-normal figures plus focused numerical regressions

Stack

Depends on #1279, which is layered 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 commits.

Review-only diff: svchb/TrixiParticles.jlOpen@surface-normals/interface-documentation...surface-normals/multicolor-thresholds

Verification

• focused surface-normal tests: 390/390
• focused surface-tension tests: 95/95
• IISPH color-contribution constructor and particle-packing dispatch checks passed
• multicolor CairoMakie figures rendered successfully with CairoMakie 0.15
• 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 89.92248% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.42%. Comparing base (a720b1d) to head (300aebe).
⚠️ 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	92.30%	3 Missing ⚠️
src/schemes/fluid/surface_tension.jl	89.65%	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    #1280      +/-   ##
==========================================
+ Coverage   90.39%   90.42%   +0.03%     
==========================================
  Files         138      139       +1     
  Lines       11322    11409      +87     
==========================================
+ Hits        10234    10317      +83     
- Misses       1088     1092       +4     

Flag	Coverage Δ
total	90.43% <89.92%> (+0.03%)	⬆️
unit	77.67% <86.04%> (+0.18%)	⬆️

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[svchb]

Coordination note: #1282 now contains the multicolor surface semantics, threshold handling, and focused regressions that overlap this draft. After #1282 settles, this PR should retain only any unique documentation/figures, or be closed if fully subsumed. No replacement stack layer is planned.