Softening model

[svchb]

Based on #1208.

Summary

Replaces the scalar softening_length API from #1208 with explicit gravitational softening models. The public softening models are NoSoftening and PlummerSoftening; the abstract softening/model hooks remain internal.

Updates the n-body example system to carry the softening model through its gravity conversion path and to compute potential energy through the gravity softening helper. The docs page and base n-body allocation/energy tests now live in #1208, so this PR is focused on the softening-model API delta.

Testing

• test/general/gravity.jl
• test/examples/n_body_system.jl

[codecov]

Codecov Report

❌ Patch coverage is 95.89041% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.04%. Comparing base (71d42df) to head (15e2e8d).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
src/general/gravity.jl	95.08%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1209      +/-   ##
==========================================
+ Coverage   89.88%   90.04%   +0.16%     
==========================================
  Files         133      138       +5     
  Lines       10452    10642     +190     
==========================================
+ Hits         9395     9583     +188     
- Misses       1057     1059       +2     

Flag	Coverage Δ
total	90.05% <95.89%> (+0.17%)	⬆️
unit	70.74% <75.34%> (+0.24%)	⬆️

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