Support Java 17 sealed classes (#66)

[paulirwin]

Fixes #66.

Translates Java sealed, permits, and non-sealed as far as C# allows.

Behavior

Java	C# output	Warning
sealed class (option on, default)	public closed class Shape	no
sealed class (option off)	public class Shape	yes
sealed interface	public interface Service	yes
non-sealed class	public class Square : Shape	yes

In every case a comment recording the original hierarchy is emitted above the type, so the permits clause is not lost:

// Java: sealed, permits Circle, Square, Rectangle
public closed class Shape
{
}

Sealed interfaces have no C# equivalent at all, so they always warn. non-sealed re-opens a hierarchy that C# never closed in the first place, so it emits no modifier and warns.

Option

New JavaConversionOptions.UseClosedForSealedClasses, enabled by default, controls whether sealed classes get the C# 15 closed modifier (which requires .NET 11). It is exposed as --no-closed-for-sealed-classes on the CLI and as a checkbox in the GUI settings window.

Notes for review

• Roslyn has no ClosedKeyword token yet, so closed is emitted via SyntaxFactory.ParseToken. Verified it survives NormalizeWhitespace().
• The Java17SealedClasses.java fixture is registered under GeneralSuccessfulConversionTest rather than FullIntegrationTests, because the latter compiles the output with Roslyn and closed will not parse until the toolchain supports C# 15. Worth moving once it does.
• Records cannot be sealed in Java, but a sealed interface permitting records is the canonical pattern. The interface side is covered here; adding the // Java: comment to permitted records in RecordDeclarationVisitor would be a follow-up.

Testing

New ConvertSealedTests.cs covers all four cases above plus the no-permits and non-sealed-type paths. Full suite passes (318 tests); solution builds clean.

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