Make SPH correction configuration consistent

[svchb]

Summary

• Introduce CorrectionConfiguration so density and gradient corrections can be selected independently.
• Apply density correction before assembling gradient moments in WCSPH and EDAC.
• Use asymmetric corrected gradients consistently while retaining conservative fluid-boundary pressure coupling.
• Extend dummy-boundary and rigid-body support and reject unsupported IISPH correction configurations explicitly.
• Add focused unit coverage and manufactured-solution convergence validation.

Validation

• Correction, pressure-acceleration, WCSPH, EDAC, IISPH, boundary, and RHS tests pass.
• validation/corrections/convergence.jl covers corrected density and gradient operators, boundary behavior, and pressure consistency.
• The complete combined-stack unit suite also passes.
• git diff --check passes.

Dependencies

This PR is independent of the generalized surface-method PR.

[codecov]

Codecov Report

❌ Patch coverage is 95.13274% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.46%. Comparing base (a720b1d) to head (4d2d21b).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/general/corrections.jl	91.07%	5 Missing ⚠️
src/io/io.jl	0.00%	5 Missing ⚠️
src/schemes/structure/rigid_body/system.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1281      +/-   ##
==========================================
+ Coverage   90.39%   90.46%   +0.07%     
==========================================
  Files         138      139       +1     
  Lines       11322    11523     +201     
==========================================
+ Hits        10234    10424     +190     
- Misses       1088     1099      +11     

Flag	Coverage Δ
total	90.47% <95.13%> (+0.07%)	⬆️
unit	78.83% <93.80%> (+1.35%)	⬆️

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[svchb]

This draft has been split into three smaller review layers:

• Fix SPH correction operator lifecycle #1284 - correction operator robustness and lifecycle
• Configure density and gradient corrections independently #1285 - independent density/gradient correction configuration
• Add SPH correction consistency study #1286 - consistency documentation and manufactured validation

The original branch remains available for reference, but this umbrella PR is superseded by those drafts.