fuse `scatter(in, ind, elem_op(gather(in, ind), update))` into scatter

```
%2 = "stablehlo.gather"(%arg0, %c) <{dimension_numbers = #stablehlo.gather, slice_sizes = array}> : (tensor<4x4xf64>, tensor<4x2xi64>) -> tensor<4xf64>
%3 = stablehlo.add %2, %1 : tensor<4xf64>
%4 = "stablehlo.scatter"(%arg0, %c, %3) <{scatter_dimension_numbers = #stablehlo.scatter, unique_indices = true}> ({
^bb0(%arg2: tensor, %arg3: tensor):
stablehlo.return %arg3 : tensor
}) : (tensor<4x4xf64>, tensor<4x2xi64>, tensor<4xf64>) -> tensor<4x4xf64>
```

The issue gives a StableHLO gather/add/scatter pattern but names no source files or tests. Search the repository for stablehlo.scatter and stablehlo.gather handling, then determine where this pattern can be recognized and verify that the resulting scatter preserves the indexed update behavior.

Written by the indexing model from the issue text.