🤖 Repo Assist — implementing feature requested by @dsyme in #570.

Summary

Closes #570. When a series or frame is printed in FSI (or via .Format()/.ToString()), cell values longer than 50 characters are now truncated with "...", preventing a single long string from making an entire column unreadably wide.

Root Cause / Motivation

The existing formatTable already does per-column PadRight alignment, but it used the raw (uncapped) cell lengths to compute column widths. A single value of length 200 would make the whole column 201 characters wide, completely destroying readability. This was reported in #570 with a working proof-of-concept.

Fix

Added two changes to Formatting module in src/Deedle/Common/Common.fs:

1. MaxCellWidth = 50 — new configurable constant (consistent with the suggestion in Make fsi print more beautiful #570 and pandas' default display width).
2. truncateCell — private helper that clips strings exceeding MaxCellWidth and appends "...".
3. Modified formatTable — builds a clipped 2D array before computing column widths and padding, so alignment is computed on the capped values.

let MaxCellWidth = 50

let private truncateCell (s:string) =
    if s.Length > MaxCellWidth then s.Substring(0, MaxCellWidth - 3) + "..."
    else s

let formatTable ((redacted) =
    ...
    let clipped = Array2D.init rows columns (fun r c -> truncateCell data.[r, c])
    // compute widths from clipped, then PadRight

The change is surgical — only formatTable is touched. FormatStrings and the rest of the pipeline are unchanged.

Trade-offs

• Values ≥ 50 chars are truncated in display only; the underlying data is unaffected.
• The 50-char limit is a constant, not runtime-configurable. If demand arises, MaxCellWidth can be made mutable (like RowStartItemCount) in a follow-up.

Test Status

✅ All 615 tests pass on Linux (dotnet test tests/Deedle.Tests/Deedle.Tests.fsproj -c Release).

3 new formatting tests added:

• series with long string values truncates at MaxCellWidth
• frame with long string values truncates at MaxCellWidth
• series short values are not truncated

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