Fix FixedNBins marginal-mode dispatch

[BJMCox]

Fixes #551.

Implement FixedNBins at the existing _get_bining_impl seam used by _get_binedges. The internal spelling remains unchanged to avoid unrelated churn.

Main code: +1/-1.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 65.83%. Comparing base (6e8795f) to head (2a1bebf).

Files with missing lines	Patch %	Lines
src/statistics/binning_rules.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #559      +/-   ##
==========================================
+ Coverage   65.45%   65.83%   +0.38%     
==========================================
  Files         121      121              
  Lines        7060     7060              
==========================================
+ Hits         4621     4648      +27     
+ Misses       2439     2412      -27     

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