Fix batch of issues found in post-merge review

[GiggleLiu]

Summary

• SubsetSum generics: Made SubsetSum<T> generic over integer type with SubsetSumElement trait, default T=i64, registered i128 variant, added positive-integer validation and new_unchecked for internal use
• 3-SAT → SubsetSum overflow: Changed target to SubsetSum<i128>, replaced hard n+m<=18 assertion with checked_mul/checked_add giving helpful error suggesting larger integer type
• IsomorphicSpanningTree complexity: Fixed from "num_vertices^num_vertices" to "factorial(num_vertices)"
• RuralPostman complexity: Fixed from "num_vertices^2 * 2^num_required_edges" to "2^num_vertices * num_vertices^2" (Held-Karp bound)
• Paper entries: Added PartitionIntoTriangles problem-def and TravelingSalesman→QUBO reduction-rule with worked examples; added 7 missing bib entries
• Skill update: Updated add-model skill to require test_<name>_paper_example unit test

Test plan

• make check passes (fmt + clippy + 1850+ tests)
• make paper compiles without errors
• Verify SubsetSum generic API works for downstream users
• Verify paper renders correctly (PartitionIntoTriangles figure, TSP→QUBO example)

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 99.37500% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.86%. Comparing base (e73796d) to head (3f1b0ee).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/models/misc/subset_sum.rs	94.73%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #642      +/-   ##
==========================================
+ Coverage   96.83%   96.86%   +0.03%     
==========================================
  Files         246      246              
  Lines       31855    32287     +432     
==========================================
+ Hits        30846    31276     +430     
- Misses       1009     1011       +2     

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