Reuse the differentiation backend

[joaquimg]

_diff is re-entered after every optimize! (which invalidates model.diff), and each rebuild instantiated a fresh LazyBridgeOptimizer, re-resolving a bridge graph that depends only on the problem structure.

The backend is now kept in model.diff_backend under a (constructor, parametric) key and merely MOI.empty!-ed on reuse, which clears model and bridge maps but preserves the resolved graph.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.86%. Comparing base (5537420) to head (64b0ff7).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #377      +/-   ##
==========================================
+ Coverage   91.82%   91.86%   +0.03%     
==========================================
  Files          16       16              
  Lines        2705     2717      +12     
==========================================
+ Hits         2484     2496      +12     
  Misses        221      221              

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