Generalize free-surface detection

[svchb]

Summary

• Generalize surface geometry behind AbstractSurfaceMethod and AbstractSurfaceNormalMethod.
• Add activity-only ColorfieldSurfaceDetection and make ColorfieldSurfaceNormal compute the same activity plus normals.
• Support multicolor fluid interfaces, free surfaces, walls, rigid bodies, interpolation, custom quantities, metadata, and VTK output through one pipeline.
• Rename surface_normal_method to surface_method; retain deprecated constructor-keyword and accessor compatibility.

Breaking change

The system field and primary keyword are renamed for the 0.6.0 release. The changelog entry is therefore under Version 0.6.0.

Validation

• Complete unit suite: TRIXIPARTICLES_TEST=unit julia --project=. -e 'using Pkg; Pkg.test()'.\n- Focused surface detection, multicolor, wall/rigid, normal, surface-tension, and interpolation tests pass.\n- git diff --check passes.\n\n## Follow-up integration\n\nThe surface-activity consumers in Add smooth Morris interface activity #1269, Add balanced CSS surface tension #1271, Add normal smoothing and VTK diagnostics #1272, Add free-surface tangential shifting #1273, and Add interface-aware tensile instability control #1274 can be rebased onto this shared pipeline.

[codecov]

Codecov Report

❌ Patch coverage is 87.45645% with 36 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.35%. Comparing base (a720b1d) to head (d23652d).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/schemes/fluid/surface_normal_sph.jl	89.04%	16 Missing ⚠️
src/schemes/fluid/fluid.jl	57.14%	6 Missing ⚠️
src/schemes/fluid/surface_tension.jl	0.00%	4 Missing ⚠️
src/general/custom_quantities.jl	62.50%	3 Missing ⚠️
src/schemes/structure/rigid_body/system.jl	50.00%	2 Missing ⚠️
src/general/semidiscretization.jl	75.00%	1 Missing ⚠️
src/io/io.jl	90.90%	1 Missing ⚠️
...rc/schemes/fluid/entropically_damped_sph/system.jl	83.33%	1 Missing ⚠️
...chemes/fluid/implicit_incompressible_sph/system.jl	87.50%	1 Missing ⚠️
...rc/schemes/fluid/weakly_compressible_sph/system.jl	88.88%	1 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (a720b1d) and HEAD (d23652d). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (a720b1d)	HEAD (d23652d)
total	1	0

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1282       +/-   ##
===========================================
- Coverage   90.39%   79.35%   -11.04%     
===========================================
  Files         138      138               
  Lines       11322    11490      +168     
===========================================
- Hits        10234     9118     -1116     
- Misses       1088     2372     +1284     

Flag	Coverage Δ
total	?
unit	79.35% <87.45%> (+1.87%)	⬆️

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