Harden kernel gradient correction

[svchb]

Summary

• guard kernel-gradient normalization against invalid coefficients
• use a zero gradient offset as the finite fallback
• preserve Float32 boundary cache storage
• add constant-field, restart, EDAC, and GPU regression coverage

Stack

Depends on #1292, which depends on #1284. This PR targets main and temporarily includes the preceding stack.

Review-only diff: svchb/TrixiParticles.jlOpen@fix/shepard-correction-lifecycle...fix/kernel-correction-lifecycle

Isolated layer: 6 files, 102 additions, 4 deletions.

Verification

• complete unit suite passed
• JuliaFormatter and git diff --check passed

[codecov]

Codecov Report

❌ Patch coverage is 86.25000% with 44 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.45%. Comparing base (a720b1d) to head (b8c594e).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
test/examples/gpu.jl	0.00%	22 Missing ⚠️
src/schemes/structure/rigid_body/system.jl	46.15%	7 Missing ⚠️
...c/schemes/structure/total_lagrangian_sph/system.jl	57.14%	6 Missing ⚠️
src/general/corrections.jl	82.14%	5 Missing ⚠️
test/general/corrections/common.jl	95.60%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1291      +/-   ##
==========================================
+ Coverage   90.39%   90.45%   +0.06%     
==========================================
  Files         138      142       +4     
  Lines       11322    11616     +294     
==========================================
+ Hits        10234    10507     +273     
- Misses       1088     1109      +21     

Flag	Coverage Δ
total	90.46% <86.25%> (+0.06%)	⬆️
unit	78.64% <83.43%> (+1.15%)	⬆️

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