Support all ObjSCIP features

[hedtke]

Closes #29

Plugins derived from SCIP's ObjSCIP base classes can now be used without touching the raw SCIP*.

API

• Model::include<Plugin>(args...) constructs and includes a plugin derived from ObjBranchrule, ObjConshdlr, ObjCutsel, ObjDisp, ObjEventhdlr, ObjHeur, ObjIISfinder, ObjNodesel, ObjPresol, ObjPricer, ObjProp, ObjReader, ObjRelax, or ObjSepa. The plugin is constructed by SCIP++ because the base classes need the SCIP* in their constructors. SCIP takes ownership, the method returns a non-owning pointer or nullptr if including failed. Pricers are activated after including them.
• Model::includeBenders<Benders>(nSubproblems, args...) includes and activates a Benders' decomposition together with SCIP's default Benders' cuts.
• Model::includeBenderscut<Cut>(benders, args...) includes a custom Benders' cut.
• Model::setMessagehdlr(std::unique_ptr<ObjMessagehdlr>) installs a message handler.
• A new Model constructor takes std::unique_ptr<ObjProbData>, a new Model::addVar overload takes std::unique_ptr<ObjVardata>.

ObjTable is not supported because SCIP does not export it from its shared library, see scipopt/scip#222.

Notes

• The kind of a plugin is selected by overload resolution on its base class. A plugin deriving from several base classes is rejected at compile time.
• ObjConshdlr plugins must be included with needscons = FALSE, as SCIP++ cannot create constraints of a custom handler. The readme documents this and the other per-class caveats.
• Every supported base class has a test with a minimal plugin, plus tests for the failure path where SCIP rejects a plugin and SCIP++ deletes it.
• The coverage upload moved to codecov-action v7, with test results uploaded as a separate report type.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.63%. Comparing base (44d6a47) to head (57437af).
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #44      +/-   ##
==========================================
+ Coverage   97.75%   99.63%   +1.87%     
==========================================
  Files          10       11       +1     
  Lines         223      274      +51     
  Branches       64       87      +23     
==========================================
+ Hits          218      273      +55     
+ Misses          4        0       -4     
  Partials        1        1              

Files with missing lines	Coverage Δ
include/scippp/model.hpp	100.00% <100.00%> (+3.70%)	⬆️
source/model.cpp	98.92% <100.00%> (+0.96%)	⬆️
source/model_objscip.cpp	100.00% <100.00%> (ø)

... and 4 files with indirect coverage changes