Fix VectorBase.java SubSequence compilation error

[Copilot]

The SubSequence method in VectorBase.java had broken code attempting to cast an uninitialized Data[] array to MutableSequence, causing compilation failure.

Changes

• VectorBase.java: Replace broken SubSequence implementation with stub that throws UnsupportedOperationException
• Add .gitignore: Exclude *.class files and common IDE artifacts

Before

public MutableSequence<Data> SubSequence(Natural start, Natural end) {
    Data[] arrCopy;
    return (MutableSequence) arrCopy;  // Compile error: uninitialized variable
}

After

public MutableSequence<Data> SubSequence(Natural start, Natural end) {
    throw new UnsupportedOperationException("SubSequence not yet implemented");
}

Original prompt

Delegate: "Delegate to cloud agent"

Created from VS Code via the GitHub Pull Request extension.

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