Add interface-aware surface pressure

[svchb]

Stack status

This draft joins two prerequisite lines:

• Correction stack: Fix SPH correction operator lifecycle #1284 -> Harden Shepard density correction #1292 -> Harden kernel gradient correction #1291 -> Harden gradient correction matrices #1290 -> Complete mixed kernel-gradient correction #1288 -> Conserve corrected pressure pair momentum #1289 -> Configure density and gradient corrections independently #1285 (28ed03b469).
• Generalized free-surface detection: Generalize free-surface detection #1282 (63807976a2).

Merge commit a94de3ca4a joins #1282 onto the correction stack. The review delta after that join is 9da1756512 plus the integration-only test fix 2f001f3e76: 15 files, 469 insertions, and 21 deletions. The validation-only #1286 remains related but does not block this PR.

Summary

• Add opt-in SurfacePressureDifference() support to WCSPH and EDAC through the surface_pressure keyword.
• Blend the configured conservative pressure acceleration with a first-order consistent pressure-difference operator according to the target particle's surface_activity.
• Restrict blending to interactions within the same fluid system; boundary and cross-fluid interactions remain conservative.
• Support GradientCorrection and MixedKernelGradientCorrection, including the asymmetric corrected-pressure paths from the correction stack, and reject unsupported configurations.
• Require a configured surface method so pressure blending uses the shared global surface-quantity lifecycle from Generalize free-surface detection #1282.
• Keep transport-velocity pseudo-pressure shifting on its conservative path with use_surface_pressure=false.
• Add metadata, documentation, examples, focused dynamic tests, and manufactured-solution validation.

Numerical behavior

The pressure-difference operator removes the constant-pressure free-surface defect. Where surface activity is nonzero, the blended operator is intentionally not pairwise momentum-conservative; boundary and cross-fluid interactions are not blended. The manufactured solution records this residual and confirms that it decreases under refinement.

Validation

• Complete unit suite: 23,729 tests pass.
• Pressure acceleration: 45/45.
• Colorfield surface detection: 39/39.
• Manufactured solution: approximately first-order boundary and second-order interior convergence; constant-pressure boundary error is zero and the momentum residual decreases under refinement.
• JuliaFormatter 2.1.1 is idempotent.
• git diff --check passes.

[codecov]

Codecov Report

❌ Patch coverage is 86.93069% with 198 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.12%. Comparing base (6a56a72) to head (2f001f3).

Files with missing lines	Patch %	Lines
test/examples/gpu.jl	0.00%	154 Missing ⚠️
src/schemes/fluid/surface_normal_sph.jl	93.78%	12 Missing ⚠️
...rc/schemes/fluid/weakly_compressible_sph/system.jl	91.66%	6 Missing ⚠️
src/general/corrections.jl	95.06%	4 Missing ⚠️
src/schemes/fluid/pressure_acceleration.jl	93.33%	4 Missing ⚠️
src/general/custom_quantities.jl	62.50%	3 Missing ⚠️
src/preprocessing/particle_packing/system.jl	50.00%	3 Missing ⚠️
src/schemes/fluid/surface_tension.jl	89.65%	3 Missing ⚠️
src/schemes/structure/structure.jl	81.25%	3 Missing ⚠️
...chemes/fluid/implicit_incompressible_sph/system.jl	89.47%	2 Missing ⚠️
... and 4 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1283      +/-   ##
==========================================
- Coverage   90.49%   90.12%   -0.38%     
==========================================
  Files         141      150       +9     
  Lines       11419    12714    +1295     
==========================================
+ Hits        10334    11458    +1124     
- Misses       1085     1256     +171     

Flag	Coverage Δ
total	90.12% <86.93%> (-0.38%)	⬇️
unit	81.53% <86.07%> (+3.70%)	⬆️

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