Fix combinatorial reduction boundary cases and witness recovery

[isPANN]

Summary

• Normalize interchangeable equal-size pairing elements in 3D matching to 3-partition witness recovery.
• Handle empty/uncovered matching instances and propagate target arithmetic construction errors.
• Preserve SetSplitting set semantics for repeated elements and singleton contradictions.
• Force negative-weight vertex-cover choices in the AND/OR graph construction and recovery.
• Update the corresponding bounds, proofs, and regression tests.
• Preserve the existing extract_solution, aggregate, and solver APIs; based on main.

Verification

• The complete extracted-change integration tree passes 6,572 workspace all-feature unit/integration tests (2 ignored).
• Constituent changed-line coverage: 100% for matching and negative-weight recovery, 97.14% for SetSplitting.
• Paper checks passed for the constituent rule changes; consolidation does not change their implementations.

[codecov]

Codecov Report

❌ Patch coverage is 99.46524% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 95.95%. Comparing base (7dd5fcd) to head (efc399d).

Files with missing lines	Patch %	Lines
src/rules/setsplitting_betweenness.rs	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1155      +/-   ##
==========================================
+ Coverage   95.93%   95.95%   +0.02%     
==========================================
  Files        1074     1074              
  Lines      132106   132208     +102     
==========================================
+ Hits       126730   126865     +135     
+ Misses       5376     5343      -33     

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[GiggleLiu]

Final review: read all four fixes and the Garey–Johnson normalisation argument; make check and make paper pass locally on the merged state. Approving as-is.