Which issue does this PR close?

Closes #62.

Rationale for this change

DataFusion's Rust ScalarUDFImpl::invoke_with_args speaks ColumnarValue (Array or Scalar) rather than raw Arrow arrays. The Java binding previously materialised every scalar arg to a length-N array before crossing the JNI boundary, which lost the scalar-vs-array distinction and forced nullary UDFs to learn the batch row count by some out-of-band channel (the workaround proposed in PR #57).

Aligning the Java API with the Rust enum eliminates the workaround: a nullary UDF can return ColumnarValue.scalar(...) and the framework broadcasts it, and a UDF that takes literals sees them as Scalars without per-row duplication.

What changes are included in this PR?

• New ColumnarValue sealed interface (Array/Scalar records, factory enforcing length-1 invariant on scalars).
• New ScalarFunctionArgs record bundling List<ColumnarValue> and rowCount.
• ScalarFunction.evaluate is now evaluate(BufferAllocator, ScalarFunctionArgs) -> ColumnarValue (source-breaking).
• JniBridge.invokeScalarUdf rewritten to ship two struct arrays (length-N Array args + length-1 Scalar args) plus a byte[] argKinds positional mask, returning a byte indicating the result variant. JNI signature is now (Lorg/apache/datafusion/ScalarFunction;JJJJ[BJJI)B.
• Native invoke_with_args no longer materialises scalars; it partitions args by ColumnarValue variant and reconstructs the result from the returned kind byte via ScalarValue::try_from_array.
• AddOneExample and docs/source/user-guide/scalar-udf.md updated; new "Returning a Scalar" section added to the user guide.

How are these changes tested?

make test — 135 tests pass (12 pre-existing skips). Existing ScalarUdfTest cases (AddOne, Concat, Square, error paths, volatility round-trip) adapted to the new signature, plus three new tests:

• nullaryScalarReturnUdf_overMultiRowQuery_broadcasts — a nullary java_pi returns ColumnarValue.scalar(...) and the framework expands it across rows, replacing the rowCount workaround.
• scalarLiteralArg_arrivesAsScalarColumnarValue — UDF asserts that a SQL literal arrives as ColumnarValue.Scalar (length 1), proving scalar-ness survives the FFI.
• udfReturningScalar_isBroadcastByFramework — explicit scalar-return path test.

Also covered by cargo clippy --all-targets --workspace -- -D warnings (clean) and ./mvnw spotless:check (clean).

Are there any user-facing changes?

Yes — source-breaking signature change to ScalarFunction.evaluate. Implementations must:

Before:

public FieldVector evaluate(BufferAllocator allocator, List<FieldVector> args) {
    IntVector in = (IntVector) args.get(0);
    // ...
    return out;
}

After:

public ColumnarValue evaluate(BufferAllocator allocator, ScalarFunctionArgs args) {
    IntVector in = (IntVector) args.args().get(0).vector();
    // ...
    return ColumnarValue.array(out);
}

Nullary or broadcast-style UDFs can return ColumnarValue.scalar(...) over a length-1 vector.