Fix #95: Add BinPacking model

[GiggleLiu]

Summary

• Add BinPacking<W> optimization model to src/models/optimization/
• Minimizes number of bins used for items with sizes subject to capacity constraints
• Config space: each of n items assigned to one of n possible bins
• Parameterized by weight type W (i32, f64) for item sizes

Closes #95

Plan

See docs/plans/2026-02-25-bin-packing.md for the full implementation plan covering:

1. Model implementation (BinPacking<W> struct + Problem/OptimizationProblem traits)
2. Unit tests (creation, evaluation, brute-force solver, serialization)
3. CLI dispatch registration
4. Paper documentation (#problem-def)

Test plan

• Unit tests for creation, evaluation (valid/invalid), direction, serialization
• Brute-force solver finds optimal solution on example instance (6 items, capacity 10)
• CLI can load/solve/serialize BinPacking instances
• make check passes (fmt + clippy + test)
• Paper builds with new problem definition

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 91.36691% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.33%. Comparing base (9671215) to head (252eee8).

Files with missing lines	Patch %	Lines
src/models/optimization/bin_packing.rs	81.66%	11 Missing ⚠️
...t_tests/rules/unitdiskmapping/pathdecomposition.rs	92.85%	1 Missing ⚠️

❌ Your patch check has failed because the patch coverage (91.36%) is below the target coverage (95.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #96      +/-   ##
==========================================
- Coverage   96.36%   96.33%   -0.03%     
==========================================
  Files         197      199       +2     
  Lines       27214    27352     +138     
==========================================
+ Hits        26224    26349     +125     
- Misses        990     1003      +13     

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