Fix #444: [Model] MinimumCardinalityKey

[GiggleLiu]

Summary

Add MinimumCardinalityKey satisfaction problem model — a classical NP-complete problem from relational database theory (Garey & Johnson A4 SR26). Given attribute names, functional dependencies, and a bound M, determines whether a candidate key of cardinality ≤ M exists.

Fixes #444

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.17%. Comparing base (08d5574) to head (8f07479).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #679      +/-   ##
==========================================
+ Coverage   97.16%   97.17%   +0.01%     
==========================================
  Files         302      304       +2     
  Lines       39923    40108     +185     
==========================================
+ Hits        38791    38976     +185     
  Misses       1132     1132              

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[GiggleLiu]

Implementation Summary

Changes

• src/models/set/minimum_cardinality_key.rs — New MinimumCardinalityKey satisfaction problem model with functional dependency closure under Armstrong's axioms
• src/models/set/mod.rs — Module registration and re-export
• src/models/mod.rs — Added to set re-exports
• src/unit_tests/models/set/minimum_cardinality_key.rs — 11 unit tests (creation, evaluation YES/NO, non-minimal rejection, bound check, solver, serialization, edge cases)
• problemreductions-cli/src/cli.rs — Added --num-attributes and --dependencies CLI flags, help table entry
• problemreductions-cli/src/commands/create.rs — CLI create arm with dependency parser
• src/example_db/fixtures/examples.json — Regenerated with new model example
• docs/paper/reductions.typ — Problem definition entry with worked example
• docs/paper/references.bib — Added Lucchesi & Osborn (1978) citation

Deviations from Plan

• None

Open Questions

• None

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