Fix #90: Add ClosestVectorProblem model

[GiggleLiu]

Summary

• Adds implementation plan for ClosestVectorProblem (CVP) as a new optimization model
• CVP: Given lattice basis B and target t, find integer x minimizing ‖Bx - t‖₂
• Parameterized by element type T (i32/f64), reuses VarBounds from ILP for integer enumeration bounds
• Plan covers: model struct, Problem/OptimizationProblem traits, module registration, CLI dispatch, unit tests, paper documentation

Plan Tasks

1. Create CVP struct with constructor and accessors
2. Implement Problem and OptimizationProblem traits (Minimize, f64 metric)
3. Register in module exports (optimization/mod.rs, models/mod.rs)
4. Register in CLI dispatch (dispatch.rs, problem_name.rs)
5. Write comprehensive unit tests (evaluation, solver, serialization, edge cases)
6. Add paper documentation (display-name + problem-def)
7. Final verification (make test clippy)

Closes #90

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.36%. Comparing base (ef2905f) to head (00ce084).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #92   +/-   ##
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  Coverage   96.36%   96.36%           
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  Files         197      197           
  Lines       27210    27210           
=======================================
  Hits        26220    26220           
  Misses        990      990           

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