test: proptest fuzz testing for variant constraint solver (REQ-052)

[avrabe]

Summary

6 property-based tests for the feature model solver (1200 random cases total):

• Solver never panics on any random model + config
• Resolved variant satisfies all group constraints (mandatory/alternative/or)
• Effective features ⊇ selected features (propagation only adds)
• Root always selected on success
• Ancestor closure: every selected feature's parent chain is selected

Verifies: REQ-052

🤖 Generated with Claude Code

[github-actions]

⚠️ Performance Alert ⚠️

Possible performance regression was detected for benchmark 'Rivet Criterion Benchmarks'.
Benchmark result of this commit is worse than the previous benchmark result exceeding threshold 1.20.

Benchmark suite	Current: 4f30d49	Previous: 7588c3c	Ratio
store_insert/10000	14845947 ns/iter (± 1126112)	12015669 ns/iter (± 777347)	1.24
link_graph_build/10000	33933713 ns/iter (± 3084386)	25885542 ns/iter (± 1621608)	1.31
validate/10000	13870148 ns/iter (± 1832476)	9578640 ns/iter (± 316048)	1.45
diff/10000	8941022 ns/iter (± 865910)	7224219 ns/iter (± 189161)	1.24

This comment was automatically generated by workflow using github-action-benchmark.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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