Add physical 3D Akinci cohesion model

[svchb]

Summary

• add the exported opt-in SurfaceTensionAkinciCohesionPhysical model for physical surface tension values in N/m
• convert the requested surface tension to the internal 3D Akinci coefficient using the planar surface-energy moment
• retain central pair forces without surface-normal storage and preserve pairwise linear and angular momentum
• use a dimensionless same-kernel wall ratio for wall and rigid-body coupling
• extend Test timestep selection and add capillary interface condition #1024 with physical-Akinci single-system and fluid-interface capillary restrictions
• add metadata output, validation, and user documentation

Stack

Depends on #1252 and #1024. It also retains the Akinci dependency chain #1259 -> #1258. PR #1024 is not currently contained in #1252, so both heads are explicit ancestors of this branch. The physical model is isolated in commit 3f84349fb; merge commits 7a364dbc2 and 024248c22 integrate #1252 and #1024, and bdb842f87 tests the physical interface timestep.

The logical dependency order is #1252, #1024, #1258, #1259, then this PR. The branch targets main, so inherited changes remain visible until the dependency PRs merge.

Normalization

For physical surface tension sigma, reference density rho_0, and compact-support radius h_c, the internal cohesion coefficient is gamma = sigma / ((21 / 7040) * rho_0^2 * h_c^2). The wall adhesion_coefficient is a dimensionless multiplier of the same kernel; (1 + cosd(theta)) / 2 maps a target contact angle to this ratio.

Verification

• surface-tension tests: 146/146
• dedicated timestep tests: 9/9
• complete unit test target passed with eight Julia threads
• restacked physical 3D smoke test passed, including zero RHS allocations
• independent quadrature recovered the requested surface tension from the kernel moment
• JuliaFormatter 2.1.1 passed

The complete examples target was not rerun for this stack layer; the affected 3D fluid and wall paths passed through the targeted example.

[codecov]

Codecov Report

❌ Patch coverage is 92.60642% with 99 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.46%. Comparing base (48920d8) to head (bdb842f).
⚠️ Report is 13 commits behind head on main.

Files with missing lines	Patch %	Lines
src/general/gpu.jl	0.00%	8 Missing ⚠️
src/general/semidiscretization.jl	94.53%	7 Missing ⚠️
...chemes/fluid/implicit_incompressible_sph/system.jl	60.00%	6 Missing ⚠️
test/count_allocations.jl	0.00%	6 Missing ⚠️
test/general/semidiscretization.jl	91.42%	6 Missing ⚠️
src/general/neighborhood_search.jl	90.00%	5 Missing ⚠️
src/schemes/fluid/surface_tension.jl	93.97%	5 Missing ⚠️
src/setups/rectangular_tank.jl	98.34%	5 Missing ⚠️
test/general/custom_quantities.jl	83.33%	5 Missing ⚠️
src/schemes/fluid/shifting_techniques.jl	33.33%	4 Missing ⚠️
... and 23 more

❗ There is a different number of reports uploaded between BASE (48920d8) and HEAD (bdb842f). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (48920d8)	HEAD (bdb842f)
total	1	0

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1260       +/-   ##
===========================================
- Coverage   90.13%   79.46%   -10.67%     
===========================================
  Files         138      141        +3     
  Lines       10797    11758      +961     
===========================================
- Hits         9732     9344      -388     
- Misses       1065     2414     +1349     

Flag	Coverage Δ
total	?
unit	79.46% <92.60%> (+3.10%)	⬆️

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[svchb]

Closing this draft because the rebuilt Akinci stack intentionally excludes SurfaceTensionAkinciCohesionPhysical and its physical-coefficient and capillary-timestep dependencies. No commits from this branch are carried into the replacement stack. The active path is #1258 -> #1259 -> #1261 -> #1266 -> #1267, with aggregate #1262 and validation #1263.