[Rule] EXACT COVER BY 3-SETS to MinimumEdgeCostFlow

[isPANN]

Source: EXACT COVER BY 3-SETS
Target: MINIMUM EDGE-COST FLOW
Motivation: Skipped — source problem X3C is a specialization of Set Covering (each set has exactly 3 elements, exact cover required). Implement the general Set Covering reductions first.
Reference: Garey & Johnson, Computers and Intractability, ND32, p.214

Specialization Note

• X3C (Exact Cover by 3-Sets) is a restriction of Set Covering where each set has exactly 3 elements and an exact cover is required.
• MinimumSetCovering exists in codebase at src/models/set/minimum_set_covering.rs.
• X3C itself does not yet have a dedicated model. It could be modeled as a MinimumSetCovering instance with constraints, or as a separate type.
• Blocked on: X3C model implementation (P129).