// Group by first element of a tuple key, use second element as inner row key
frame |> Frame.nestRowsBy fst snd
// → Series<'K1, Frame<'K2, 'C>>

// More explicit example
let df =
    Frame.ofColumns [ "V" => Series.ofValues [10; 20; 30; 40] ]
    |> Frame.indexRowsWith [("North","Jan"); ("North","Feb"); ("South","Jan"); ("South","Feb")]

let nested = df |> Frame.nestRowsBy fst snd
// nested.["North"].RowKeys = ["Jan"; "Feb"]   ← clean inner key
// nested.["South"].RowKeys = ["Jan"; "Feb"]
```

## Implementation

- **`FrameModule.fs`**: `nestRowsBy` defined after `nestBy`; calls `nestBy groupSel` then applies `indexRowsWith (Seq.map rowSel)` to each inner frame.
- **`FrameExtensions.fs`**: matching `NestRowsBy(frame, groupSelector:Func<_,_>, rowSelector:Func<_,_>)` C# extension.
- **`tests/Deedle.Tests/Frame.fs`**: one test covering the tuple-key case.

The change is **purely additive** — `nestBy` is unchanged.

## Test Status

```
Passed! - Failed: 0, Passed: 670, Skipped: 0, Total: 670