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Closes #9.

What

Adds Frame.compare — the frame-level counterpart to the already-existing Series.compare. Given two frames, Frame.compare returns a new Frame<'R, 'C> where each cell is a Diff(obj) describing the change between the two frames.

let df1 = frame [ "A" =?> series [ 1 => 1.0; 2 => 2.0; 3 => 3.0 ]
                  "B" =?> series [ 1 => 10.0; 2 => 20.0 ] ]
let df2 = frame [ "A" =?> series [ 1 => 99.0; 2 => 2.0; 4 => 4.0 ]
                  "C" =?> series [ 1 => 100.0 ] ]

let diff = Frame.compare df1 df2
// Column "A": key 1 → Change(1.0, 99.0); key 2 → (omitted, unchanged); key 3 → Remove 3.0; key 4 → Add 4.0
// Column "B": all rows → Remove (column only in df1)
// Column "C": all rows → Add   (column only in df2)

Approach

• Root cause: Frame.compare was never implemented, even though Series.compare and the Diff<'T> type have existed for a while.
• Fix: In FrameModule.fs, iterate over the union of both frames' column keys, retrieve each column as Series<'R, obj> (using TryGetColumn(obj) — columns missing from one frame yield Series.empty), then delegate per-column comparison to the existing Series.compare / series.Compare(). The result columns are collected into a new frame via FrameUtils.fromColumns.
• Why not a Frame member? Adding Compare directly to the Frame<'R,'C> type body (which uses F#'s mutually-recursive and with FrameUtils) caused type-inference issues that constrained FrameUtils.fromColumns to obj, breaking unrelated code. Implementing it as a standalone function in FrameModule.fs avoids this entirely, consistent with how strConcat and zip are structured.

Test status

All 615 existing tests pass. Three new tests cover:

1. Detecting Change, Remove, and Add values in shared columns
2. Columns present in only one frame
3. Identical frames producing empty diff columns

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