Fix SetSplitting reduction for repeated elements

[isPANN]

Summary

• Deduplicate subset elements before decomposition so repeated entries retain set semantics.
• Encode deduplicated singletons with contradictory Betweenness constraints, preserving infeasibility.
• Preserve the existing construction, evaluation, and witness-extraction APIs. This is an independent correctness fix based directly on main (3 files).

Verification

• Regression test reproduces the constructor panic for [0, 0, 1] before the fix.
• Repeated-element tests check source/target feasibility and witness recovery, including unsplittable singleton sets.
• Full workspace all-feature tests: 6,479 passed; 2 existing ignored.
• Focused reduction tests: 6 passed.
• Changed-line coverage: 97.14% across 35 lines.
• Formatting, diff whitespace checks, and workspace all-target all-feature Clippy with warnings denied passed.

[codecov]

Codecov Report

❌ Patch coverage is 97.14286% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 95.93%. Comparing base (7dd5fcd) to head (5707218).

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

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1157   +/-   ##
=======================================
  Coverage   95.93%   95.93%           
=======================================
  Files        1074     1074           
  Lines      132106   132140   +34     
=======================================
+ Hits       126730   126764   +34     
  Misses       5376     5376           

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[isPANN]

Consolidated into #1155 for one review of combinatorial reduction correctness fixes. No changes have been merged to main.