[FEA] Support streaming ``shift``/``diff`` expressions in cuDF-Polars

[rjzamora]

Row-order-sensitive expressions such as plain shift() are supported by the GPU engine in single-partition/in-memory execution, but are not supported by streaming multi-partition execution.

This issue should cover the common row-order expression variants that need boundary state across streaming partitions:

• shift
• diff
• pct_change
• forward_fill
• backward_fill

Example:

def test_expr_shift_streaming(engine: GPUEngine) -> None:
    """``shift`` on the full frame.

    Status
    ------
    - In-memory OK
    - Streaming NOT supported
    """
    lf = pl.LazyFrame({"x": [1, 2, 3, 4, 5]})
    q = lf.select(
        pl.col("x").shift(1).alias("lag_1"),
        pl.col("x").shift(-1).alias("lead_1"),
    )
    assert_gpu_result_equal(q, engine=engine)