Implement array.flatten builtin

[ume3445]

Summary

Implements array.flatten as described in #143.

Changes

• Added ArrayFlatten to ArrayBuiltins.java, following the same pattern as the existing array.concat/slice/reverse builtins
• Flattens exactly one level of nesting per the compliance test fixture, non-array elements pass through unchanged
• Throws a TypeError matching the existing message format when the operand isn't an array

Testing

• Ran the full compliance suite locally: all 6 array/flatten cases pass
• Full opa-evaluator suite passes with no regressions (2096 tests, 0 failures, 0 errors)

Closes #143.

[sspaink]

Thanks!