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Implements the long-requested Frame.interleave function from issue #187.

What it does

Given a list of frames with the same column keys, Frame.interleave places them side-by-side, tagging each frame's columns with its 1-based position in the input list. The result has a 'C * int column index.

let df1 = Frame.ofValues [("x","a",1); ("y","a",3); ("x","b",2); ("y","b",4)]
let df2 = Frame.ofValues [("x","a",5); ("y","a",7); ("x","b",6); ("y","b",8)]

Frame.interleave [df1; df2]
// Columns: [("a",1); ("b",1); ("a",2); ("b",2)]
//
//      ("a",1)  ("b",1)  ("a",2)  ("b",2)
// x ->    1        2        5        6
// y ->    3        4        7        8

Rows are aligned with an outer join, so frames with disjoint row keys also work.

Changes

• src/Deedle/FrameModule.fs — adds Frame.interleave : Frame<'R,'C> list -> Frame<'R, 'C * int> after Frame.merge in the "Joining, merging and zipping" category
• src/Deedle/FrameExtensions.fs — adds FrameUtils.Interleave(seq(Frame<'R,'C)>) C# extension method
• tests/Deedle.Tests/Frame.fs — adds 6 tests:
  • Column key shape (tuple (colKey, 1-based-index))
  • Value correctness for a 2-frame case
  • 3-frame case (tags 1, 2, 3)
  • Single-frame identity (columns become ('c', 1))
  • Outer-join row alignment (missing values for non-shared rows)
  • Empty-list guard raises ArgumentException

Implementation notes

The implementation is minimal and composes existing primitives:

let interleave (frames: Frame<'R, 'C> list) : Frame<'R, 'C * int> =
    if List.isEmpty frames then
      invalidArg "frames" "The list of frames must be non-empty."
    frames
    |> List.mapi (fun i f -> f |> mapColKeys (fun c -> (c, i + 1)))
    |> List.reduce (fun acc f -> acc.Join(f))
```

This is essentially the workaround I described in my #187 comment, promoted to a named first-class function.

## Test Status

```
Passed!  - Failed: 0, Passed: 696, Skipped: 0, Total: 696

6 new tests added (690 base → 696). Full build (dotnet build src/Deedle/Deedle.fsproj -c Release) succeeds with 0 errors.

Closes #187

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