Add interface-aware surface pressure

[svchb]

Stack status

This draft depends on #1282 and the correction implementation layers #1284 and #1285. The validation-only #1286 is related but does not block this PR. The current branch still contains the pre-split dependency commits because the correction and surface-method changes overlap in the fluid-system constructors. After the dependencies merge, this branch will be rebased so the review delta is only commit f16070658 (14 files).

Summary

• Add opt-in SurfacePressureDifference() support to WCSPH and EDAC.
• Blend conservative pressure acceleration with a first-order consistent pressure-difference operator using the target particle surface_activity.
• Restrict blending to same-fluid-system interactions; boundary and cross-fluid interactions remain conservative.
• Support GradientCorrection and MixedKernelGradientCorrection and reject unsupported configurations.
• Keep transport-velocity shifting on its conservative pseudo-pressure path.
• Add configuration, metadata, documentation, focused dynamic tests, and manufactured-solution validation.

Numerical behavior

The difference operator removes the constant-pressure free-surface defect but is intentionally nonconservative where activity is nonzero. The validation records the expected momentum residual and confirms that it decreases under refinement.

Validation

• Complete unit suite passes.
• Pressure acceleration: 39/39.
• Focused dam-break correction matrix: 42/42, including both supported surface-pressure corrections with zero RHS allocations.
• Manufactured solution: approximately first-order boundary and second-order interior convergence; constant-pressure boundary error is zero.
• git diff --check passes.

[codecov]

Codecov Report

❌ Patch coverage is 90.21739% with 54 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.33%. Comparing base (a720b1d) to head (f160706).
⚠️ 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/io/io.jl	57.89%	8 Missing ⚠️
src/general/corrections.jl	89.28%	6 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	40.00%	3 Missing ⚠️
.../schemes/boundary/wall_boundary/dummy_particles.jl	90.47%	2 Missing ⚠️
src/schemes/fluid/pressure_acceleration.jl	94.44%	2 Missing ⚠️
src/general/semidiscretization.jl	75.00%	1 Missing ⚠️
... and 3 more

❗ There is a different number of reports uploaded between BASE (a720b1d) and HEAD (f160706). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (a720b1d)	HEAD (f160706)
total	1	0

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1283       +/-   ##
===========================================
- Coverage   90.39%   80.33%   -10.06%     
===========================================
  Files         138      139        +1     
  Lines       11322    11729      +407     
===========================================
- Hits        10234     9423      -811     
- Misses       1088     2306     +1218     

Flag	Coverage Δ
total	?
unit	80.33% <90.21%> (+2.85%)	⬆️

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