ssa: handle union types in type conversion

[zhouguangyuan0718]

Summary

• teach goTypes.cvtType to convert *types.Union values recursively
• cache converted union types the same way as other composite types
• add a regression test covering a degenerate union interface used as a value type

Testing

• go test ./test -run TestVerificationKeyUnionDegenerate -count=1

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Codecov Report

❌ Patch coverage is 0% with 18 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
ssa/type_cvt.go	0.00%	18 Missing ⚠️

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